I spent 4 years on and off studying for the GMAT. I would study for a month get distracted by something else and then take a two month break. Start studying again for another month, get distracted again, and then take another break. All in all I probably spent a good 12 months studying for the GMAT. Of course most of the time I spent studying was re-learning/re-reading many of the subjects that I forgot.
The last time I took the GMAT I spent a good 5 straight months cramming for the GMAT (cramming = 20+ hrs/week). I would suggest that 3-4 months is probably sufficient for most individuals, of course, your study time could vary depending on your desired score (higher score = more study time).
Learning at a slower pace may actually help you retain the material better than someone who whizzed through the book. My suggestion is to read through everything slowly the first time to get a firm grasp on everything and then whiz through the book a second time through.
I would highly recommend postponing an October test date if you do not think you will be ready in time. You can still apply to MBA programs next year if you take the test in January/February.
