I agree with Karishma, but I'd like to push the point a little more. 700 is a very difficult score to get. Everyone wants that score precisely because most people *never get it.* Sure, we see 700+ scorers all the time, but getting there from 460 is no joke. I have met students who have been studying this test for years trying to hit that goal!
If you are serious about getting 700, you should probably start with a class or tutoring. If you are rusty on your basics, you may want to precede the class with a month or so of self-study. For instance, we offer a 500-page Foundations of Math book that takes you through all the basics you need to succeed with the more difficult material in our course (and in the Official Guide).
As your class progresses, be sure to take periodic practice tests to see how close you are to your goal. This will help you determine how much additional study will be needed. You may find that you need to take the real GMAT repeatedly to hit your desired score. In the end, though, it's hard to say what your experience will be. Perhaps a great deal of long-unused knowledge will come flooding back to you as you study, in which case the process might not be *that* long. Just be prepared for a fight--if you want to outdo 90% of test-takers, just be aware that most of those people are working really hard to get a great score.