The amount to prep might best be determined by the student's curiosity and interest in spending time prepping.
In our case interest was fairly high, so the house was strewn with library books for a month or so. It seemed to be a combo of "Whoa, look, math AND pretty bubbles to fill in" plus a way to sort of assuage questions of what the scores would be, what the test would look like, etc, plus satisfying innate curiosity. However, it was apparently not very useful in practice (techniques like skipping a lot and coming back, and whatever nonsense was up with the reading section, for example, fell by the wayside) and its main benefit was entertainment in the form of new math concepts, reading passages, and witty remarks on behalf of the Kaplan test prep books.
I would say that the prep did raise the scores to the highest possible AT THAT TIME, if you know what I mean -- it raised familiarity, but not ability really. So in that way I think it made the scores a more accurate portrayal of ability than just going in blind.
