Sleep is something that's highly variable from person to person, there's no "right" amount of time to sleep, only what's right for you (obviously this is a little bit of simplification, I don't think anyone could function on 1 hour of sleep a day for very long). 8 hours is generally thrown around as a good benchmark, but some people may only need 5 or 6 while others need 9 or 10. It depends on your metabolism, general health, and a variety of other factors I'm sure.
I find with exercise and by sticking to a very rigid sleep schedule (I've learned that for me, It's very bad to fight the sleepies to "just get a little more work done"), 6 is generally enough for me personally. This depends on a nice solid sleep though, if something is periodically interrupting my sleep (like a noisy air conditioner or what have you) I can sleep for longer and still not feel terribly rested. I find oversleeping too much also tends to make me groggy. YMMV
I find with exercise and by sticking to a very rigid sleep schedule (I've learned that for me, It's very bad to fight the sleepies to "just get a little more work done"), 6 is generally enough for me personally. This depends on a nice solid sleep though, if something is periodically interrupting my sleep (like a noisy air conditioner or what have you) I can sleep for longer and still not feel terribly rested. I find oversleeping too much also tends to make me groggy. YMMV