I would say 3 is a max for me. First sem, I had 5 hours in a row (either 11-4, or 12-5 on a tuesday) and by the end of it, my brain was like mush. I have a very short attention span, so that just might be me.
also, it might depend if it's lectures or tutes. if it was 4 hours of lectures in a row, i'd probably fall asleep, since it's mostly a passive activity.
gloam:
If you move out of a full class, it goes to from full to, say, 93%. If somebody doesn't change into that class, then you can go back to that class. If you move from an overfull class, it goes from overfull to full and you can't get back in.
With the morning classes thing, I think the earliest classes and the latest classes have the lowest amount. There were over 100 macro streams last sem, and the ones that seemed the most empty were the 8am and 6pm ones. If you're a morning person, then that sounds good. I knew somebody who had to go into uni all five days (which is kinda funny for a comm student) and he timetabled everything as early as he could so he could have time to do stuff after all his classes finished.