Asking how long you need to study for the GMAT is like asking how long you need to train to run a marathon in under 2:30.
If you're already at 3:15 and have built up a decent base, it might only take a few months of focused training. Likewise, if you've run a 3:30 without seriously training that demonstrates some untapped potential that will make it easier for you to run faster with some effort. If your last race was 4:00, you're going to have to work a bit harder to get there. Some people, regardless of how hard they train, just don't have the genetic makeup to ever get there.
Still, no matter where you start from, you'll benefit from taking things gradually, being consistent with your training, and ramping up difficulty as you go. Same deal with GMAT prep.