I stay up far too late most of the time; in many cases I do not even go to sleep until it is light out. I keep telling myself I'll start going to sleep earlier, but I rarely do. I can reliably make myself go to bed early is if I have to get up early the next day for work or some other commitment, or if I am horribly sleep deprived, but in most other cases I simply cannot bring myself to care enough to do so. In my defence, however, I have mostly been working the night shift lately (so even if I go to bed at 8am, I can still sleep for nine hours), and I enjoy having privacy, which is probably one reason I stay up so late many nights.
Back when I had school I would generally stay up until about 2am. When I was working the morning shift I would usually go to bed at around 10 or 11pm.