i would aim for 4 weeks. i've heard stories about people starting off with a 27 on AAMC 3, and then 4 weeks later ending up with a 36 on their last exam (and AAMC 3 is supposedly the easiest). point is, the practice just makes you more comfortable with the format. so you want to make sure you give yourself time to adjust to it. even if you bomb the first one, don't get discouraged. this annoying computer shi* just takes some time to get used to