We normally recommend 2-3 months of GMAT prep, although the time will vary based on the hours you dedicate. I advise you to visit MBA.com and take a free official GMAT practice test to see what you are currently scoring. This will give you an insight into how many points you need to improve upon on the test.
Regarding your second question, some MBA programs all you to choose majors, whiles others graduate all students with a general degree (Harvard Business School does this for example). You can pursue part-time, full time, or executive programs. Some business schools also offer online classes.
I hope this was of help.
Conrad and the Stacy Blackman
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