Re: GMAT
You need atleast 4 weeks of study.
I did in Jan last year. You need to practice for maths questions always looking for 'short cuts' since you are pressed for time. 'Short Cuts' being alternative ways to solve the same question quicker.
Practice the english section... the most important thing is to read the answers to the practice questions and review the reasons why the choices were WRONG rather than concentrating on right answers. After a while, those reasons would start to repeat and you'll get a feel for most of the 'tricks' the GMAC plays in the verbal section.
Good Luck!