Elementary school, I remember that I went off to Japan for two weeks and did all the homework for those two weeks the weekend before I left. It was all pretty easy stuff.
High school, about four hours a night. It was usually okay, though I was perfectionist and I could get pretty worked up over math or physics problems. Weirdly at my school we were allowed to use answer keys sometimes, but you didn't get credit unless you could show step-by-step how you got the answer. It was pretty useful to know if you got the answer right before turning in the assignment, because then you could keep trying until you did it right, which made the homework a lot more useful.
College was pretty easy. I sometimes gave myself homework to practice stuff that was hard or that I knew would be difficult to do under a time limit.
Grad school... lol. I didn't even buy the books for most of my classes. I think I've averaged around four homework assignments per quarter. And only the quarters I've taken classes. The hardest grad school class I've been involved in is the last one I TA'd because I had to write the exams. And that's way harder than taking one. Then again, for my option, I only have to take six classes total.