Spent about 1.5 months studying for the GMAT in my off time during senior year of college. Took my first practice test and ended up with a 660 on a dry run, really suffering in Math. To turn it around I got GMAT Pill (shameless promotion) and sunk a week into each section of the test, a little more time than they recommended. Overall the experience was pretty good, I was scoring a lot better on math (their frameworks for data sufficiency are excellent for helping someone who isn't used to those types of questions), and my verbal scores were pretty consistently in the 90%'ile range. While my verbal score didn't really change from first practice test to last, I did feel like I improved a lot, as I was no longer guessing based on what sounded most correct to me, but rather I knew the rules and concepts behind my answers. (Yes GMAT Pill I recognize that last sentence was an X/Y consistency error and probably should be revised).
I found myself exhausted with studying by the end of the month and a half as I had poured a ton of hours into the program and had done hundreds of practice problems, so I would have suggested 1-1.25 months instead. But overall, the program is a great way to pace yourself through the material and found the GMAT pretty easy after.