Study until your practice tests are coming back in a range you are comfortable with. That will mean different amounts to different people. Personally, I studied 1-2 hours a day outside of the class starting 2ish months prior to the test, and ramping up as the test approached an additional hour or so a day. The last week before the test I studied about 8-10 hours a day, with the major focus being on practice tests, particularly in the physical sciences (where I was scoring worst). I did well. Others who studied with me in the same schedule did considerably less well. AAMC practice tests are a very good guide to how prepared you are.