The answer to this question, as you would expect, depends on what semester you want to apply for. Going by estimated deadlines, if you are applying for the Fall, I would recommend that you write GRE at the latest by October. I say this because this gives you enough time to concentrate on the other parts of the application which are almost equally, if not more, important than GRE scores. If you are applying for Spring, you should have finished with the GRE by June at the latest. Make sure that you are aware of the available test dates at the center where you plan to take the test and schedule the test well in advance.