The Physics GRE takes a bit of preparation, where "a bit" is approximately 6-12 months. The test is based on speed, therefore preparation for this specific test is of the essence. If you are pressed for time, you should also focus on those topics that have the most impact, see Wiki Physics GRE page. For example, there are 6 questions for laboratory methods and data analysis. For more ways to study, go to Alex Lang's website