If you are working then i guess 3hrs of serious preperation on weekdays and 5-6 hrs of serious studies on weekends will be enough.
You would take 3-4weeks to get adjusted to the routine and to get into the top of your concentration level.
After that i guess 3months should be enough for the rpeperation.
Regarding teh gmat preperations i have written a blog on the same. You cna find the link in my signature.
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