It really can vary from person to person, but most people would need a minimum of 2 months to achieve that kind of score improvement - possibly 3 or 4.
How have you done on standardized tests in the past? Have you performed in the 90th percentile? (700 is 90th percentile.) Did you study on your own? How easy / hard was it and how much time did it take you? Did you take a class or work with friends or a tutor? Your experiences on past standardized tests (assuming you've taken any!) can help you to estimate how much time you might need this time around.
Also, probably you haven't taken any CAT-based standardized exams in the past - most tests are still paper-based. If that's the case, and if you do plan to study on your own, then it is VERY important to educate yourself about the differences we see in CATs and what implications those differences have for timing, how we take the test, how we study, etc.
A good starting point is your GMAT Uncovered guide in your MGMAT student center. You can find it in the Class Resources section of your student center.
Also, you might find this article helpful:
http://www.manhattangmat.com/blog/index ... ice-tests/And these two if you work on your own:
http://www.manhattangmat.com/blog/index ... tudy-plan/http://www.manhattangmat.com/blog/index ... our-study/And, of course, don't hesitate to come back here and ask questions! :)