It's just a rule of a thumb. Everyone's different. I studied for 6 months and I'm glad I did. Four months into my prep my practice test average was 33. I was aiming for 36 and with only a month to go until my exam I chose to delay by a month and a half in hopes of bringing my score up. My AAMC 8-11 average (taken in the last month of my six months of studying) was 37. My score on the real thing ended up being 39S (same as my AAMC 11 score, but with a different breakdown). There is no way I would have gotten that 39S or anywhere close to it if I had listened to people and taken my exam when I had originally planned.
There is a caveat though: you have to handle long term studying differently than short term studying ("short" being a relative term here). With a 3 month schedule like SN2ed's you can afford to blaze through content without ever stopping to look back. The idea is that you cover material so rapidly that it's all still fresh in your head by the time you take the exam. With long term studying though you have to stop to constantly review what you've already learned so that you don't forget any of it, and you need to do lighter amounts of studying per day than you would with a 3-4 month schedule since you're definitely going to burn out otherwise. For some people (such as myself) this works out well since you get spaced repetition that enhances your knowledge of the subject, and on a schedule that makes it easy to balance studying with work/life. For others though, this strategy works out poorly because they mismanage their schedule and blow through materials on a particular topic early on or never stop to review, and thus forget everything by the end. It really depends on what your needs are, and what strategy works best for you.
Just don't get carried away if you decide to go with a long term schedule. This is an exam where you're never going to feel truly ready, and there's going to come a point where you need to recognize when you're at your peak and stop there before you start to decline in performance.