Plan and take the GRE General Test about a year prior to your expected entrance into graduate schools. Preferable time would be during summer or early fall of the year you are applying to colleg. The deadline for GRE will vary significantly depending on the admission process in each school to which you apply. You need to look at the application guidelines to find out whether you need to take Subject Test in addition to the General test. GRE Subject tests are offered only three months (November, December & April) in a year, so consider their schedule into your timeline.
Since the GRE General Test is allowed up to 5 times in a year and once in 30 days cycle. The year is calculated from July 1 to June 30 as per the US calendar. Keep these in mind while planning to take GRE General Tests. See to avoid re-taking the test unless you think this will only increase the scores considerably.
If you take GRE early, you will have your score in hand and can use it to plan your application strategy and to choose and apply to a variety schools in which you will be viable. Taking GRE early allows you to submit your scores along with your application.
Taking GRE General Test 8 weeks before your application submission deadline gives you enough time to complete other task like Essay writing, Letter of Recommendation, Resume, etc. Also, the official scores for the GRE General Test take approximately 10 – 15 days from the test date. GRE Subject test takes 6 weeks usually from the test date to get the scores.