Try to spend no more than 2 hours reading or studying without a significant break. Your brain checks out after 2 hours. My butt checks out before that! Some brains check out earlier, but it's a rare brain that can take more than 2 hours straight. Even better - take a break every hour. And don't forget to eat!
This is why MOST college lecture courses are in ONE hour segments - the lecturers want you paying attention. High school classes are even shorter, because teen-agers have a shorter attention span.