Well, it can vary. However, I read before I go to sleep which really helps me focus and clear my mind of everything that is happening during the day. It varies because if the book is good and I'm really tired I can stay up reading for 4 hours!! !! However, some nights I've been so active or just tired, I'll be out in 15 mins. I read until my eyes can't stay open which can be 10 mins to 4 hours. But if I'm under a lot of stress, worrying about too much, or nevous about the next day, I'll stay up reading for 2 to 4 hours.
The key for me is once I fall asleep, I sleep all night and have a hard time getting up in the morning. I have always been a night person and not a morning person which cause problems when I go to sleep at 4am and have to be up at 8am. (But I seem ok with getting up.) Plus, I can't go to sleep before 11pm because I'm too awake and have too much energy. Also, I take a shower before I go to bed to relax me.