I basically study until I understand all the day's material and have finished my homework. Then I take 15 minutes to go over the material for the next day (if we have a syllabus that tells us that). That way when I come to class I'll understand the in-class material better since I've already seen it the night before. It will depend on what kind of classes you're taking. It always takes me more than an hour to do math homework, but no more than an hour for English since I am better at it (unless I have an essay due).
If you're taking four classes and are studying more than 5-6 hours a night (this is a lot for all but the most dedicated students), though, then I'd go ask your teachers for advice on what to do and how to lighten your workload. It's hard to give advice since I don't know your strengths/weaknesses.