It depends on how far you are from achieving your goal scores and how far you are from achieving your goal scores. It is recommended that you do not study for the GRE for less than 2 weeks and, even then, you will likely reach diminishing returns by the time you have studied for 6 months. The ideal time to study is somewhere between one month and five months, depending upon how much time you have available. Gradeway Prep GRE coaching in Kolkata is the best place to start.