Ideally, you should be studying for the MCAT with every class you take, learning the concepts in a way that you will retain them for years. But in a pragmatic sense, starting about six months before your target test date should give you plenty of time to complete your review and take practice exams.
I'd suggest reviewing for one to two months on your own with a general review book. Then start hardcore practice passages about four months before your test date and try to average 10 passages/day (thoroughly reviewing every answer). With about six weeks to go before your MCAT start taking practice CBTs (ideally two to three a week) for five weeks. Score them until you are semi-happy and consistent in terms of scores. STOP SCORING your exams and just do them for the experience once you feel okay. This way, if you get that anomoly low score, you won't panic.
Spend the last week getting good sleep and putting the final touches on your test-taking strategies and information.