Personally, I think it's never too early to study for the MCAT. I wouldn't suggest studying intensely ... but there is a LOT of material and anyone who attempts to master the material will realize time becomes an obstacle. If you familiarize yourself with some of the concepts in advance (concepts you encountered in class already but perhaps weren't taught very well at your institution), you could save yourself a great deal of time in the future. It also wouldn't hurt to brush up on some concepts you already learned so that you won't forget them in the future. I know for me personally, I forgot a great deal of information. I took a Physics class in the summer and there were so many important concepts that I just couldn't remember. What I wish I did while in college, was get an MCAT prep book so that way I can focus on MCAT topics only.
Also, most people on here seem to struggle with the verbal portion of the MCAT. If you think verbal might be difficult for you, try reading online journals. Try to get comfortable reading about unfamiliar subjects. Analyze what the author is saying. What message is he trying to give, if any? Also, it wouldn't hurt to increase your vocabulary. Why not consider learning a word a day?
http://wordsmith.org/words/today.html
Anyhow, just like those said above, the most important form of studying for the MCAT is your college pre-reqs. The more you understand the material taught in those classes, the easier your MCAT experience will be!! Trust me. You'll save yourself a whole lot of unnecessary stress if you learn it well the first time.
Aside from brushing up and trying to keep the material fresh in your mind, I would allocate about 3 months to adequately prepare for the MCAT.
Goodluck!