I studied for 1.5 months the last time I took the MCAT, but I had also taken the MCAT before. If this is your first go around I would suggest 2.5 to 3 months. But each individual is different, if you have a solid backgroung and retained most of the stuff from your pre-med classes you may only need 1-1.5 months. But that would be constant studying for a month. Not just on the weekends, so this probably only applies to those who are taking the August MCAT.
I think it mainly depends on how comfortable you are with the material. The more comfortable the less time needed to prepare.
Good luck....