How do you know how much to study?

The key is raising the targeted skill. Do the homework and either study to raise the skill or raise the skill in the regular ways. For example if the targeted skill is handiness you can either study or work on the crafting bench. The game does not seem to care how the skill is raised so long as it is raised. If you have maxed the skill prior to attending Uni then all you need to do is the homework, term paper and presentation if they are also required. Having said this I am basing this on my Sim living at home. Things might be a little different living on Campus.