Official Practice diagnostic test assesment

Hi guys, I got this on Gmat official CAT practice test w/o any prep: V37 Q 47 with a total score of 690. I have few questions: (1)How much time do I need for prep if I want to touch 750. (2) For my Quant 47/51 the percentile shown is 59, I don’t understand why the percentile is so low. (3) Lastly, h...