I think you may be asking the wrong question. Hours of prep aren't nearly as important as quality of prep. You could be studying less than an hour a day but making great progress if you stick to a good study strategy/schedule.

In general though, I would say that an hour per day for 2-3 months is appropriate for your prep. You can read about my old GMAT prep strategy in my original GMAT blog found here: https://beatthegmat.blocked