I had only about 10 days to prep. Gregmat is the absolute GOAT. I cannot possibly overstate the importance of the vocab mountain on there, although I only did about 28 groups properly. I went through the practice tests on ETS, Magoosh, and Gregmat. I wasn't very hopeful about a 320, I was scoring around the 315 range on most practice tests, and I messed up time management a lot on all of those. Guess I lucked out on exam day or I just work well under pressure :D. I needed a 320 for Tu Delft and NUS and a couple of other unis I was looking at and I'm super happy about my score.
I did mess up a few questions in the actual exam because I wasn't paying attention but if you're tight on time to prep, I'd be happy to help answering any questions you might have.