For me, it depends on what time I have to go to work the next day. If I have to go in early then I try to go to bed early. If I go to work later in the day or if I don't have to go to work at all the next day, then I can stay up a bit longer.
I go to sleep around midnight - 2am typically during the summer. I was staying up til 4am for about 3 weeks. That was getting a little tough on me. I'm enjoying just 2-3 right now. But I'm about to have to fix my schedule for work again.