I started accumulating review books etc. around the summer before my junior year, but I honestly didn't look at them until much later. I took the April MCAT during my junior year, and I started to study during Winter Break of my junior year. That gave me four good months to procrastinate! I started off really excited to jump into my studying, I got bored once the Spring Semester started, and then a month before the test it was like a race to the finish. Overall, I got about a good month's-worth of studying. I guess some people need about a year's worth of reviewing, but I did just fine with my month's-worth.
One thing I would not recommend: don't wait til the last minute all together! I know someone who waited to study until the last week before the 2002 MCAT. Needless to say, her scores were....hideous (to say the least).
Don't start so early that you get disillusioned with the whole thing before you even sit for the test, but don't wait til the last minute. Just know your own study habits and needs and go from there.
