This is a very tough question, as you are not giving us any information concerning your background...
I think the most important thing to do is to evaluate your strengths and your weaknesses.
- Do you have a good quantitative background ?
- Are you native English speaking ?
- Are you good in reading ?
- ....
Some people will study for the GMAT for 3 weeks and they will get an easy 700. Some other people will need probably a year and a lot of practice to get the 700. All depends on your background, your general knowledge and your target for your GMAT score.
What I will do is to take one of the POWERPREP TEST that you can download on mba.com and to investigate the score that you can get before starting any preparation. By the way, this web site is very usefull...