First I would like to stress that different people learn differently. Some do better by studying on their own, others do better by going to a course that gives strategy but not too much detail since they know the material well, and yet others need the strategy and someone to re-review the material with them so that it becomes fresh in their mind. For this reason, there are several courses that stress the different categories of students, as well as non course methods of studying.
The five best resources for MCAT studying:
Princeton Review course material
Kaplan course material
Exam Kracker's Material
The Berkley Review material
AAMC material
What I would stress NOT TO DO, is to use Barrons or some of the over the counter books that are making the test easier than it actually is. Because that will give you a false impression of the test.
There are several different sources for practice tests:
AAMC 3r, 4r, 5r, 6r, 7 and 8
Exam Kracker's 1g, 2f, and 3g
Princeton Review's Free ONLINE MCAT exam
that should be enough for self study because that is about 10 exams there.
Plus, you can always get the Exam Kracker's 16 mini MCATs book as well, which has one hour exams. This is just meant to use as practice; not really a score predictor. Its more like a workbook.
These are the best ways to go about it if you don't want a course and are confident you know the material well.
If you think you need more structure and discipline then take a course. But remember that you need two things: good teachers and the ability to put more time outside of class going over everything again. The class should only serve for the purposes of review and going over strategies. The rest is up to you, with or without a course. in the end, it depends on how much you study. it is important to review why you got answers wrong on diags, and in topical tests or workbook problems. It is important for you to realize how good or bad you are at standardized tests and realize for yourself how much work you'll need to improve. Don't assume that just taking the course will guarantee a higher score. If you want the guarantee, you got to put a lot of work into it outside of class time.
For some people, a month will be sufficient to study, and for others there will be a need for 2-4 months to study. Just do what works best for you.
The one last thing I would like to say is, that once you pick a company's material to study from, don't try to mix it with other company's methods. Meaning, don't use Exam Kracker's method on one passage, and TPR's method on another passage, or you'll totally confuse yourself. Once you pick a method, stick to that method if you wish to see a score improvement.