Time needed to prepare for GRE varies from student to student, but it generally takes 2-3 months in order to achieve a satisfactory score. If you are an international students, you may need extra time to get proficiency in English language.
2-3 months of rigorous practice is sufficient for GRE Preparation, but it entirely depends on the particular student and number of quality hours devoted per day for study.
However there is no alternative of practice. Practice, practice, practice until you get a very high GRE score. There are various books and online programs that can help you to achieve this goal along with self – study, hard work and determination.