Well, it depends! 
Age, activity, genetics, environment, mental strain & stress, can all determine one's optimum rest time. 
And, of course, it can change from day-to-day
However, the National Sleep Foundations 'general' guide provides a good reference.
Click below for the full report.

Interestingly, the work of world-renown 'Sleep Doctor', Dr Michael Breus, suggests people fall into one of four chronobiology when it comes to their ideal circadian rhythm.
Dr Breus studies suggest the world's population fall into defined categories:
Dolphins: 10% 
Lions: 15-20% 
Bears: 50%
Wolves: 15-20%
Ultimately, while most us are 'Bears', it's wrong to believe everyone will perform their best bouncing out of bed, hammering away till  3 pm, then, start unwinding mentally & physically and be tucked up ready for 8 nutritious hours of sleep from 10 pm.
Click to determine 'your' chronobiology type;
In no particular order;
1. Missing Breakfast - for most, minimal breakfast or a poor breakfast will disrupt digestion & metabolism later in the day and night!  The best scenario and one that may require practice is; a hearty 'good' fat breakfast, a little more of the same for lunch, and light appropriate carbs of dinner.
2. Heavy Evening (and late) mealEvening meals consumed within 3 hours of sleeping, and loaded with heavy (hard to digest) meats, will, likely, lead to disrupted sleep.  Ideally, your brain would like to switch off digestion, not, rev it up!  Heavy meaty meals might make it difficult falling to sleep.  Beware!
3. Blue Lights - Sensitive sleepers beware!   The last 20 years have seen an increase in bedroom electronics. Mobiles, PC's, Fans, TV's, Fire alarms, and security sensors has annoying lights on the rise.  Sleeping brains require dark places, and, the sight of lights sparks waking hormones into action.
4. Emotional Stress  - From personal challenges to focussing on work tomorrow, if pre-sleep thoughts are focussed on fight or flight, bingo, again, all the wrong hormones kick into gear.  This may offer sleep in phases 1 & 2 (see more on this later), but will likely drive active brain waves around 1 am.  This will cause dosed waking preventing quality Sleep Phase 3&4.  There are strategies to overcome these challenges.
5. Starting too hot, and finishing too cold - Our body temperature changes, systematically, throughout sleep.  And, male & female temperatures can offer vast differences.  Preparing for these changes is critical in assuring quality sleep.
6. Snoring - Now, for both; the offending snorer, and the poor sod dealing with the tones of a beached whale, there presents a sleeping problem.  If the 'snoring' pitch becomes deafening and wheeze-like, it could be a sign of low oxygen.  A Physician would be the right place to start.  As for the lighter sleeper, well, from earplugs (blutack) to hypnosis courses, there are options – see The Dream On Project.
7. Minimal sunlightTo maximise sleep benefit, most of us require direct sunlight throughout the day, with just 5 minutes hitting the spot.  In balancing our circadian rhythm, sunlight drives hormonal harmony, and thus, stimulates the best sleep.  See The Dream Project for tips.
8. Fresh Brain & Body - If your brain hasn't been asked to drive complex work, and, similarly, your heart, muscles & bones, have performed little effort, then it's likely there is no need for rest.  Thinking, and exercise (structured or incidental), drive good sleep.
9. Dust mites - here in Australia, dust mites rule.  Sheets, and particularly pillowcases should be washed weekly.   Pillows, also, should be aired in direct sunlight routinely and retired yearly.
10. Frequent travel - From varying time zones, planes, changes in beds & cuisine, travelling usually disrupts one's circadian rhythm.  See Jet-Lag tips in The Dream On Project. 
ATHLETES BEWARE!  Athletes training 'hard', and by that I mean, 2-3 hours + of high-intensity training sessions, can expect difficulty sleeping.  While you'd expect training-induced exhaustion and tiredness driving knock out sleep, continuous neural firing from long training sessions amplifies adrenalin, noradrenalin & cortisol.  It can take up to 8 hours for these hormones to be sufficiently buffered.  See 'The Dream On Project' for counter sleep tactics.  
1. Amplified cortisol and other stress hormones - driving change in behaviour.
2. Weakening & vulnerable immune system - undermanned T-Killer cells activating cold & flu.
3. Appetite Imbalance - namely; insulin imbalance leading to poor pick-up choices.
4. Increased inflammation - hello annoying joint and tendon niggles.
5. The decline in thermoregulation - particularly feelings of cold extremities.
Do any of these sound familiar??
○ Reduced cardiovascular output (reduced fitness).
○ Reduced muscular strength (slumping posture).
○ Depressed (reduced management of challenges).
○ Intolerant and easily angered.
○ Respond to stress with little thought.
○ Lowered sexual interest.
○ Increased interest in sugar hits.
○ Sniffling or irritating head colds.
○ Require caffeine to function.
○ Flakey skin & dry lips.
○ Sore eyes around 3 pm.
Low interest in exercise.
○ Warm temples and oily scalp.
○ Annoying joint or muscle niggles.
○ Weight gain around the belly.
○ Athletes suffer from upper-respiratory infections.
Phase 1 - 5-15 minutes
◦ Light Sleep and still aware of a presence.
◦ While nerves are starting to down fire, it's common to experience the odd neural jolt, or kick!
◦ Airways are still very much open and snoring is unusual.
Phase 2 - 5-15 minutes
◦ First a drop in body temperature and heart rate.
◦ Eyes stop moving and brain waves are transitioning to delta waves.
Phase 3 - 3 hours
◦ Brain waves switch solely to Delta waves resulting in a deep sleep.
◦ Postural muscles loose integrity as nerves switch off.
◦ Phasic (breathing muscles) also reduce firing resulting in some people subconsciously searching for air - enter; snoring.
◦ Body repairing starts while building speed towards phase 4.
Phase 4 - 3 hours
◦ Much the same as Phase 3, only more pronounced.
◦ Body temperate drops further still.
◦ You should find it difficult waking up during these phases.
◦ Should you find yourself waking up in Phase 4 - 'Consider the Dream On Project'
Phase 5 - 1 hour
◦ Rapid eye movement and likely a time for dreams.
◦ You'll now start to move around a little.
◦ Fat burning will start to slow with the introduction of blood sugar. 
◦ Finally, cortisol will now be reintroduced and we start again.

6 am

⁍ 6 am marks the release of the hormone, cortisol, which triggers your brain and body into action.
⁍ The 'fight & flight stress response produces cortisol, and, while an extended production of cortisol can lead to ill-health, a small, timely hit is critical.
⁍ Along with cortisol, another key wakening hormone, Vasoactive Intestinal Polypeptide (VIP), sparks into gear providing an increase in heart rate, breathing rate, blood vessel widening, and importantly, drives glycogenolysis. This process amps up a much-needed spike in blood sugar charging up your action for the day.
⁍ VIP also promotes bowel movements, with, at this time of day, being best for such practice.
⁍ I've found about 300mg of magnesium and a strong black coffee (about 150mg caffeine) does the trick should your pipes be somewhat unresponsive.

8 am

⁍ Between 7 -8 am your body should receive a hit of the hormone, ghrelin.
⁍ Ghrelin is responsible for making you feel hungry. Consuming a 'nutritious' breakfast is vital in resetting your circadian clock.
⁍ Key point; Ignoring the feelings caused by ghrelin will likely mess up your metabolic & sleeping cycle later in the day.
⁍ Sleep deprivation will also drive up ghrelin. Meaning, tiredness drives hunger, and usually for all the wrong choices [namely; sugar & trans fat].
⁍ A fitting breakfast will assist in nutritious night-time sleep.
⁍ Importantly, exercise, performed at 'your' relative 60-70% of max sustainable effort, for most people, is best at this time. Pushing over one's current fitness level will influence sub-optimal appetite, hydration, and ultimately metabolism, effecting night-time sleep quality.
⁍ The optimal time of day for 'harder' athletic sessions is between 2-5 pm, that is, for those appropriately conditioned.  The body sits at an ideal temperature, optimal blood flow, and prime mental tolerance for harder effort.

9-10 am

⁍ Here comes the sun - direct sunlight is now King in activating your healthy best circadian rhythm.
⁍ As little as 5 minutes will drive a perfect balance of cortisol & serotonin, which, control much of a ticktock circadian clock.
⁍ Typically, one's mood, behaviour, and attitude dips in the shorter winter days.
⁍ Coffee, green tea, and adaptogenic herbs can mimic 'some' of the sun's good influence.
⁍ Europeans are now using a device called The Human Charger (15 minutes a day) which sends a light through the ear towards the brain.

10 am

⁍ Sex hormones peak, which seems an odd time of the day, but hey, if it's on, it's on!
⁍ It's important to note, a healthy circadian rhythm promotes a healthy sexual interest.
⁍ So far, if you've exercised at a controlled intensity, grabbed a few rays, visit the bathroom, and perhaps got lucky! You're on 'circadian' course for a great sleep.

11 am - 2 pm

⁍ You're now on high alert. Concentration is at full tilt.
⁍ Alpha Waves, neural oscillations, drive mental clarity.
⁍ Now is the time to write your #1 hit song.
⁍ Performing your most complex intellectual tasks should be performed now!
⁍ Save all your easy stuff for later in the day.

2 pm

⁍ Around 2 pm signals your peak muscle reaction, coordination & balance time.
⁍ Nerves are firing best, muscle, tendon & ligament temperature is optimum, and brain waves are still sharply ranging between alpha & theta.

5 pm

⁍ Around 5 pm marks your optimum cardiovascular efficiency, best body temperature, muscle repair, protein synthesis, and workout-recovery capability peak.  For elite well-conditioned athletes, this is an even better time of day to exercise, especially if your workout is intense.
⁍ However, for those less conditioned, exercising at this time, particularly in the warmer summer months, can prevent the adequate body cooling required for optimum sleeping conditions.
⁍ I recommend a minimum of 4 hours post evening training sessions before hitting the sack - for best sleeping conditions.

6-7 pm

⁍ With the sun setting you can expect your blood pressure to be at it's highest.  The resting heart rate is highest and blood vessels restrict. So, in the event of a frustrating 5 pm meeting, don't test your blood pressure!
⁍ Sunset also promotes the hormone, leptin. Leptin's job is to break down stored body fat.  This fat provides energy for the next 12 hours - including sleeping time.  The role of leptin prevents hunger pangs as you sleep.
⁍ It important to note; Nighttime (blue) lights and a big heavier meat-based evening meal can limit leptin release - and thus, disrupting optimum sleep (and metabolism). Also, late-night (sugary) snacks with your head slapped up against a bright screen is a circadian rhythm nightmare.
⁍ The fructose factor; Some fruits, like Kiwi fruit, loaded with fructose (real sugar), consumed in the evening will help your sleep. You should feel sleepy about 20 minutes after consumption. The addition of fructose won't disrupt insulin or leptin enough to influence poor sleep. A little fruit just before bedtime is typically a good idea.
⁍ Leptin should continue to elevate, suppressing your appetite and ripping into your fat reserves - up till 3-4 am.
⁍ I also found 200mg of magnesium before bed contributed to fat loss and better night sleep.

9 pm

⁍ Around 8-9 pm your body temperature will be at its highest - pending exercise! This is why, classically, (heavier) males, kick bed linen, but blanket hog in the early hours of the morning.
⁍ You should experience heavy-set eyes sometime between 9-11pm.
⁍ This is the hormone melatonin kicking into overdrive.
⁍ Melatonin slows the heart rate, lowers adrenalin & cortisol, lowers blood pressure, lowers the body temperature, and should shut down your gastric system. The last thing your circadian rhythm needs during sleep time is being broken with bathroom stops.

11 pm

⁍ Around 11 pm, leptin should crank up, so, if you'd like to reach your ideal weight, be asleep when Santa rocks up.
⁍ Around 12-1 pm melatonin should shut down the brain, allowing for optimum sleep. Increasing delta brainwaves should be driving the sleep phases, 3-4. This is key towards top sleep and recharging.
⁍ 1-2 am should see the boost in HGH (Human Growth Hormone) central.  HGH is responsible for repair.  No deep sleep - no repair - this is a problem.

2-5 am

⁍ Between 2 am - 6 am your core temperature falls most drastically.
⁍ This is the time for ultimate neural rejuvenation, which, sometimes, can present muscle cramping (typically in the calves and feet). A pinch of Himalayan salt in water before bed can help.
⁍ Perhaps most importantly, around 3 am your T cell [killer cells] gain strength and numbers. This is key for continued immune wellness.
⁍ Also, this is the time your anti-inflammatory system kicks into overdrive. If you're carrying an injury, you want to be asleep at this time.
⁍ Around 4 -5 am your bodies core temperature is at it's lowest.  Prepare for this time with additional blankets etc.  You don't want to miss out on this ultimate sleep driven repair time.
⁍ Cortisol starts kicking in around 5.30 am, which raises your temperature, and bingo, you start again.