Deciding how long you need to study involves a lot of factors: When are your application deadlines? How much are you trying to improve your score? Are you already familiar with concepts on the GRE? Have you not taken math since high school? etc.
Generally, though, we recommend that you study for 1-2 hours a day for about 3 months (and a bit more on the weekends). That should give you enough time to thoroughly cover all the topics you will see on the exam :)
For more detailed advice about how long you should study for the GRE, check out this blog post: