The GMAT is an endurance test. Part of your "time required" involves training your brain to stay focused for over two hours. Final Thoughts
Take an official GMAT practice test before you buy a single book. If your baseline score is 550 and your goal is 720, you are looking at a significantly longer road (likely 150+ hours) than someone starting at 650. Your Quantitative vs. Verbal Proficiency time required for gmat preparation