How do you know how much to study?

Each class has a targeted skill. When you study you raise that skill, but its not highlighted in the skills tab. You can also raise the skill the regular way and that seems to count as studying. What studying does do is raise the skill if you don't have the means to do it. That is, you can raise the logic skill by studying and you don't need a chess board.

Regardless the objective is to raise the skill because that is what the game measures for your grade. Now what I don't know is how much you need to raise your skill for the grade you get.