It is
extremely difficult to score a 750; only 2% of all test takers score 750 or higher. That doesn't mean you shouldn't go for a 750, but I just want to make sure you're aware of how very difficult it is to get that score. Even the very top schools in the US have averages of "just" 700 to 720.
Studying is only partially a function of the amount of time you spend, and there isn't any amount of time that could guarantee you a 750. What's more important is the quality of your study - are you doing the right things and studying in the right way? Use the syllabus you have to know what to do each day and use the articles below to help you learn the best way to study:
http://www.beatthegmat.com/a/2009/09/23/evaluating-your-practice-testshttp://www.beatthegmat.com/a/2009/10/09/how-to-analyze-a-practice-problemhttp://www.beatthegmat.com/mba/2010/02/09/developing-a-study-planYou can review the full list of articles here:
http://www.beatthegmat.com/mba/manhattan-gmat