If you have taken all the classes, then a good rule of thumb is at least 3 months of intensive study prior to the exam date. Three months is good because you'll be able to retain much of what you began to learn from the beginning. If you have a serious problem area like for me it was chemistry, then review that only for one month before the 3 months of studying begin. Make no mistake, you will have to study for it so while studying those 3 months be SERIOUS AND DILIGENT about it. Disregard any posters on this board that claim that they hardly studied for it at all or just 2 weeks before and got a 36. Assuming that they are not lying, remember that they are the exception and not the rule.
Good luck!