I began loosely prepping in early August of '06. This consisted of doing a few exercises (10-20 questions) and reviewing concepts.
Sometime in September, I tried to secure an early/mid December sitting for GMAT. Unfortunately (or maybe fortunately), it was all booked and the next time available was January '07. I completely stopped. I lost motivation and watched baseball.
After not prepping for almost a month, I realized I did NOT want the GMAT lingering over my head during the holidays, so I decided to get serious. I scheduled on the second week of Oct. for the Saturday following Thanksgiving. That gave me exactly 7 weeks. During most of these 7 weeks, I was studying intensely, 2hrs/day and 1 cat on sat + 2hrs reviewing on Sunday.
In all honesty, I think that's the only way to go. Ideally, I would like to have practiced some more, maybe 9-10 weeks. I was peaking towards the end of my prep.
You have to get the correct mixture of prep length and intensity to maximize your studies. Setting the date is the first step.