Hi idkhur,
First of all; a 640 is a fantastic start - great job! I like that you are planning ahead for your GMAT prep and not waiting until the last minute to begin your studies. As far as overall study time, I’ve seen students need as many as 300+ study hours to achieve a 720+ GMAT score. Although you already scored a 640, since you are aiming for a 750, to be safe, you might consider allocating at least 300 hours to prep for your GMAT.
Before beginning your GMAT studying, you may consider mapping out a study schedule, to keep organized and efficient throughout your prep. When devising such a study plan, you must remember that GMAT is so challenging in part because you are presented with only 37 quant questions and 41 verbal questions on any given GMAT, yet those questions come from a huge topic pool. For instance, when presented with GMAT Sentence Correction questions, you are not just being tested on the basics of grammar; rather, you are expected to apply rules on misplaced modifiers, subject-verb agreement, idioms, parallelism, and more. Similarly on quant, when presented with Geometry questions, for example, you are not just being tested on the basics of Geometry, such as the area of a square or triangle; rather, you are expected to be able to tackle questions involving inscribed shapes, shaded regions, similar triangles, arc length, and more.
Thus, with so much to learn, you want to strive for linear and targeted learning. In other words, you want to master one topic prior to moving to the next.
For example, if you are learning about Percents, you want to make sure you learn as much as possible about Percents: percent less than, percent greater than, variable percents, and percent change. Then it will help to do numerous practice problems on Percents (50 or more). After completing the problems, do a thorough analysis of those you incorrectly answered and fix any issues needing more attention. Once those issues have been addressed, move on to the next quant topic. Ideally, you want to follow a similar process for the verbal topics: Sentence Correction, Critical Reasoning, and Reading Comprehension.
When selecting a study resource, be sure that it allows for such focused learning and practice. If you plan to study on your own for the GMAT, you may consider an online self-study course. Self-study courses typically provide detailed study plans and have granular analytics, so you can easily track your progress as you move through the course. By being able to track your progress, you will remain more engaged, and you’ll be able to more accurately forecast when you’re ready to take your real GMAT.
I also wrote a detailed article that explains some more steps that you can follow
to help achieve your GMAT score goal..
Once you begin your study please keep us updated and reach out with any further questions that pop up. Good luck!! You can make this goal happen!!