I've seen people with very strong grasps of the foundational sciences study for the MCAT in a month, while others take almost a year. I'd say the average and "golden" time is 3 months. If you're able to study full time, I'd say 2 months is enough but if you're a student then you might want to take a summer + semester.
The MCAT tests gen chem, gen bio, gen physics, orgo, biochem, psychology, sociology, and statistics.