There is no exact answer to your Q, an ideal preparation period differs from person to person and also on the basis of how much time you intent to put in on a weekly basis.

Personally, i had a 10 week prep period spread out over 4 months (8 weeks closest to my exam, 2 weeks much prior). I put in roughly 2-3 hrs each work day and almost 75% of my weekends. Its advisable to start cutting down on going out/socializing as your date approaches.
Something i cannot stress enough : Book your date as early as possible, basically the idea is that you can "drive" better if you can see the "road" and the "horizon". ("Drive" here meaning your efforts, "Road" standing for the methodology/path used, and the "horizon" giving you an idea of what lay ahead...)
My prep certainly heated up once i booked my date Hope this helps!

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If you can score a 42 quant on mgmat with out prep- bows to you. With a few months your quant will be near perfect. You should obviously work on your quant but focus on your verbal heavily as well.