If I'm allowed to fall asleep when I'm tired and wake up when I'd like? 9-10 hours.
What I normally get when I have to be at work in the morning? Anywhere from 6-8 hours. And, after after a few nights of that, when I finally have a chance to not wake up to an alarm I normally sleep around 11 hours.
I do not function well at all on anything less than 8 hours multiple nights in the row.