I can get by on 2-3 hours if need be, but I'll usually try to get at least 6 hours for the right balance of 'having time to get stuff done during the evening' and 'getting enough sleep to actually not be tired'.
If I didn't have to get up, go to work, get stuff done and generally be a responsible functional adult, I'd sleep as much as possible. I like to aim for at least 10 hours a night during weekends and holidays, 12 hours is preferable. My personal record is 17.5 hours, which would've been much more but a certain person came in to wake me up on the grounds that I (apparently) needed to eat.