Hello Emily - Thank you for your question. I am glad that you plan your graduate study in advance. I would recommend you to first find out the graduate application due dates of your interested program/s and the schools. It is better to take the GRE exam at least 2-3 months before the application due dates. Because If you are not satisfied with your score or your program requires higher score than your earned score, you have enough time to retake the exam. However, be plan to prepare well and plan to earn highest score as much as you can in your first effort. It will save your time and money. However, in case, if you have to retake, then you have enough time to prepare more before the due dates if you take the exam 2-3 months before the due dates. You do not want to worry about missing the due dates.
Good Luck for your future studies!