~7 hrs is the ideal sleeping time... But ofcourse there is nothing known as sleeping too much.
I am cranky as all hell when I sleep more than what I've standardized (five to six hours), but that can usually be attributed to the fact that I did not wake up when I was intending to.
On the topic of sleep, I think I should go see a doctor. I've, pretty much always, had a problem with staying awake during classes on which I do not do anything other than sit. And no matter how well I've slept before classes, I almost always start falling asleep, no matter how interesting the topic of (for example) the lecture is.