First Aid for USMLE Step 1 is your ace in the hole...your dental classmates will call you stupid, but you'll be surprised how much it helps. I bought a copy from '01 for around $5 at the used bookstore, and it worked fine for me.
Regarding time spent studying, it's not a question of how many weeks you invest but how you spend them. Upperclassmen friends who did well said the best indicator of success is your performance on the old exams, so I took each of the released exams back to the 1970s under "testing" conditions. This means giving yourself the correct amount of time (1:45 per section, I think?) and minimizing breaks and goofing off. I didn't trust the unreleased exams -- as they are unreleased, the answer keys aren't very reliable. Going through the decks, focusing on those topics on the cards I wasn't very strong in (and reviewing these areas in the USMLE book, my class notes, and Kaplan and outlining these in new study notes), and sitting for each of the old exams took me 5 weeks.
Try not to get discouraged, take a few days off somewhere near the middle of your study period, and "reward" yourself often and you'll do great. Avoid burnout at all costs!