I like to get about seven hours. Sometimes eight if I'm "sleeping late." Any more than that, and I have a tendency to get really bad headaches. (I actually get headaches over just about anything, and sleeping too much is just one of those weird random things that cause them.)
With seven hours, I won't feel at all tired the next day, and I'll be wide awake within five minutes of waking. I can function off of four or five, but I'll end up majorly napping the next afternoon and throwing off my entire sleeping schedule for, possibly, weeks. I am really bad at that.