I second bb. Personally, I thought about taking the GMAT a while ago (approximately 5 months back) and registered on this forum, took a free practice test (Not GMAT Prep) and kinda assessed where my scores were, and just answered questions on the Math forum
because my Quant score was pretty high without preparation. This was during the summer. Once I got back to school, I decided I was going to start focusing on my preparation, took a GMAT Prep and really, now that I think about it, though I was constantly in touch with the GMAT, I've only seriously prepared for like 3 weeks till now. And I will be spending 10 days after finals intensively preparing for it before I take the exam on the 29th. Then again, this depends on your background - I'm an engineering student and math was never really too much of an issue. My verbal was decent enough too.
I would say that spending 2-3 months with full concentration > 6 months, because you might just forget some stuff unless you're really keeping up with it, which can get hard, considering it's your last semester