I scored a 700+ in the GMAT, first attempt. I was lucky and had good advice so I feel I should contribute
It took 3 months/ 10 weeks. Btw, if you're planning to go to b-school, you'll also need another 8 weeks for all the research about schools and essays. DO not research schools until you have your scores because this test is no joke and if you score less than 650 forget the top schools.
Week 1 - Register on
http://www.mba.com (the official website of the GMAT), and order books
Recommended materials a) Princeton Review's Cracking the New GMAT b) The official guide to the GMAT latest edition c) Kaplan 800 (Just the Test CD's if you can)
Week 2 - Read the Princeton Review. This book is the best for understanding the logic of the GMAT and the shortcuts. Its very verbose but really useful
Week 3 to 6 - Study the official Guide, and solve problems in all the sections.
Week 7/8 - Start taking the official practice tests. I recommend 1 every other day. WHen I took the test, the CD had 8 tests. Also start practicing writing the Analysis essays quickly. Each test takes 4 hours, so if you work, you may need to wake at 3 am.
Week 9/10 - ONLY IF YOU ARE SCORING OVER 650, start taking the Kaplan 800 practice tests. Not the book, just their test and just the 800 tests. GMAT is an adaptive test. That means If you do well in the GMAT, you will spend most of your time facing the hardest problems so the Kaplan 800 practice prepares you for that. IF YOU ARE SCORING BELOW 600, find an easier private test CD. Whatever you do not retake the same test, you will get false sense of your performance.
Take the test immediately after this regimen. Which means book the date way in advance, probably once you are done with the official guide.
Good luck