THe answer is 'it depends.' if you are a cram and jam kind of learner, then a few weeks of practice tests or classes can help, but if you are one who is more methodical and like to learn more deliberately over time, then you might want to budget several months of prep. I feel the best prep is to take a course designed specifically to help you learn how to analyze the types of problems on the test and to deduce correct solutions, not courses that try to teach you test taking tricks to try and game the system. There is also some good software out there. I found the official GMAC study materials to be very helpful since it emulates the actual test and gets you familiar with the format on a computer. Prep courses are definitely worth the money in my opinion, though, especially if you find it difficult to be disciplined in how much and what you study. A well-designed prep course will outline the strategy for you.