What's the total prep hours for GMAT ?

Hi all, What's the advisable total prep hours for GMAT ? I work full-time and I'm single, no kids (yehey!); I plan of devoting at least 2 hours of full-time study on 4 weeknites and 6 hrs on Saturday for the next 3 weeks. Continuous (well.. at least that's the plan). I am confident with my math and ...