Therefore, taking into account the three weeks for schools to receive grades, the person in the example would have to start preparing for the GRE 16 weeks before the first application deadline (9 weeks to prepare for the first exam + 4 weeks to prepare for a new test + 3 weeks for the first test to be schools receive grades). So all inclusive, do your first GRE 3-4 months before the school deadline and start studying 2-3 months before that. That's your simple answer to when to do the GRE. In a perfect world, you should give yourself four months to prepare for GRE, but many people allow a window of only two or three months of study.
In any case, your preparation period must be determined by deadlines. For example, if your application is due in early December, you must complete the GRE by November at the latest.