I took my part I 4 years ago, so hopefully my advice will still hold true.
In general, studying the decks is plenty enough to guarantee a 90 on the test, granted you know everything in the back of those cards. Some of my classmates studied 1 month, but I studied for 3 months (putting in about 3-4 hours a night initially and finishing off with 8hr a night). Also, make sure you do practice questions from the old tests - they serve as indicators to see whether of not you have retained any info. I tried looking through kaplan's book, but I noticed that there were a lot of errors in there. I used other review books just as references.
I kicked ass on the test with basically old tests and dental decks. Good luck to you guys.