hi, i think that score is definitely achievable within the time period you'd need. my advice is to book a date as soon as possible, and then work your study course around that date. also, 6-8 hours per day seems a bit excessive. you might reach a period of time where you no longer retain any information, and studying this much may actually be counterproductive. i can't recommend exactly how many hours per day you should study, but what i can definitely say is to not study to the point where fatigue becomes an issue. maybe 3-4 hrs per day?
in addition to the
GMAT club tests, other recommended materials include:
MGMAT guides (particularly sentence correction),
official guide 11 (or
official guide 12), and of course the gmat preptest from mba.com.
i think in your case, your best bet would be to focus on the quant basics. a score of 39 in Q is fairly low, and with a few weeks of drilling down on the basics and with your timing, you should have no troubles moving up a few points (and past 650 with ease). however, don't neglect the verbal during your studying. i think the fact that you didn't touch your books for a month is bad, but there's no reason why you can't study diligently during the next few weeks and pick up your score.