I began studying about six months before my MCAT. I took the live online Kaplan course that ran from December to May. It ended a week before my test.
I waited until February before my first full length, but after that did one a week until the last week or two, then scaled it up to two a week about two weeks before the test.
Prep classes alone do not do much, you have to review the material, take practice tests (all 5 hours timed, with breaks). In fact, I often got bored in the class and did other things. However, the prep material and practice tests are well worth the cost. You want to simulate test day and get used to the length, feel, interface, etc.
I had a day by day planned schedule, and I always did a VR passage every day, and did a practice section every couple days. The Kaplan QBank is awesome, as are their flash cards. This may seem excessive, but it is worth it to get a great score and be done after one try.
My Kaplan diagnostic score was a 24, and with persistent practice tests I was getting solid 32s-36s on my practice tests, and ended up with a 35 (12 BS, 12 PS, 11 VR). So study a little every day and take section tests and full lengths. Also, be willing to review concepts you are not doing well on.
If you do Kaplan, they keep track of what topics you are weak on, what type of passages you do not do well on, etc. so you can really hone in on what needs the most work.
I should also add that while prepping I was taking Physics II and Organic II while working full time and commuting an hour away to school twice a week or more, so it can be done while in school and working. I did have the advantage of being not far removed from the courses that contained the relevant material though.