Plan on roughly three to four months of dedicated study, somewhere in the range of 300 to 500 hours if you are aiming above the average score. That is more than the AAMC's often-cited figure, but most high scorers put in more. Anchor everything to the official AAMC materials and take full-length practice tests under real, timed conditions.
The single most important rule: only sit for the real exam once your practice full-lengths are consistently near your target. A rushed test you have to retake costs you months and money. On timing, the classic path is to take the MCAT near the end of junior year or during a gap year, and no later than the spring of the year you apply, since applying early in a rolling cycle is a real advantage.