I think too much time is just as bad as too little time.
Perhaps the way to gauge when to start preparing is by considering your level of background knowledge. I would suggest evaluating your strengths and weaknesses by doing an MCAT simulation exam cold turkey. From there, you can make a realistic departure date for your studying. For instance if you're scoring above a 10 on the physical science stuff, then i'd say you don't need to spend a couple of months studying that material. If you totally fail that section, then you'll need to budget more time to "Learn" the material rather then giving your brain a refresher. Since the MCAT is only testing basic science, there is no need for overkill studying (which i'm sure a lot of you will do anyway). The MCAT is not a pure evaluation of your knowledge, since if it were, then they would give you as much time as you need to write the sections. The MCAT also tests your knowledge indirectly through applications. That said, it's important to develop exam writing savy and stamina in addition to just studying the material. This is done by having MCAT simulation exams spaced through out your study timeline. I think 3 months is ideal if you have done well in the background courses. If you're deficient in some sections then you should invest more time. As you develop your plan, make sure to include adequate time off for mental rest. Studying when you're not completely focused and alert is of little use. Don't overdo it.
Good Luck with your MCATs,
Peter