It's certainly possible, I think, to be prepared enough to do relatively well on the test if you study for a month. I studied for about five weeks, while working full-time, and made the exact score I was shooting for. I think that optimally, I would like to have studied for more like 8 weeks, in order to maximize my score, but I had time constraints.
I think it's kind of hard, though, for others to tell you how much time you need. People study at different paces, and people also start out with different levels of knowledge of the material tested. I, for example, felt that I did not even need to study biology (microbio major), and I felt like I just needed a good brush-up in orgo, but I felt completely lost in physics and I needed to devote significant time to gen chem as well. I know that others are much stronger in physical sciences, while others have only taken their pre-reqs within a year or so of taking the MCAT, and need much less review. It all depends on what kind of score you're shooting for, and where you're at when you start studying.
I would recommend devoting a significant amount of time the last couple of weeks before you take the test to taking full-length, computerized practice tests. If you are consistently making a score that you are satisfied with, take the test. If you're not, however, you should definitely wait.
