How many hours to practice for GMAT?

I usually recommend about 2 hours per day, 5 days per week, for two or three months. Most people find that is sufficient, assuming they are studying the right way and using good materials. Add a practice test per week to that, too.

I don't think anyone can say that if you study X hours, your score will improve Y points. Some people study for a few hours and show huge improvements, while others may struggle for a month without improvement. There are too many personal variables, such as your natural ability on standardized tests, your background in grammar and math, your starting point, etc, etc.

Good luck!