I studied for the GMAT for about a month.
I started witht he GMAT prep test 1 and scored a 750
Then took the Manhattan GMAT
and scored a 680
At this point I was doing 10 problems a day from 2 minute prep (great stuff to get you used to questions
2 weeks in I took a Kapklan test and scored 640. I was discouraged by this and thought the verbal was a lot harder than GMAT Prep.
This whole time I was doing problems from OG and felling more confident every day.
3 days before the test I took the 2nd GMAT Prep test and score a 720 (49Q 40V I think)
On the quant I got 8 wrong, and 6 of those were Data Sufficiency problems.
So for the last 2 days I did DS problems from OG. This helped a lot and I was able to get through problems much quicker.
Final score on test day 50Q 44V- 760. Very happy with that.
I would suggest practice, practice practice, since speed is of the essence especially in Quant. Also do the AWA when you are practicing at least once.