Sleep is overrated imo, and it also strongly depends on the person. Some people can actually be fine with 4 or 5 hours of sleep while others need 10 or 11. Like Swell said, the average is around 7-9. The reason I say it's overrated is simply because it's not practical for many to get 8 or 9 hours of sleep. I'm usually @ work before 6:30 and up at 5:30, however I simply cannot go to bed before 11:00pm. Look at pro's, I highly doubt that with their hectic travelling schedules that they get 8 or 9 hours sleep. No doubt you need sleep to recover and grow, but the reality is that some just can't get it and I grow fine on just 6.
What's more important is a consistent sleeping pattern which allows you to fall asleep faster and spend more time in deep sleep (the recovering sleep).
The way to test how much sleep you have is to go to sleep at whatever time and don't set your alarm. Whatever time you wake up naturally at is approximately how much sleep you need. Do this 2 or 3 times for a more accurate number.