Time needed to prepare for GMAT varies from student to student, but it generally takes 2-5 months in order to achieve a satisfactory score. If you are an international student, you may need extra time to get proficiency with English language.
However there is no alternative of practice. Practice, practice, practice until you get a very high GMAT score. There are various books and online programs that can help you to achieve this goal along with self – study, hard work and determination.