I would plan on 2-3 months, but don't schedule your GMAT until you've gotten far enough into your prep to see if that is the right amount of time.
Some people start by taking a practice test cold to see how they do, but without any prep, you're just wasting a test and not getting a decent baseline anyway, if you know nothing about the GMAT. Get a good prep book (I used Cracking the GMAT
and would recommend it; great overview of the test), and read through it, do some practice questions. Also get the Official Guide, 11th edition
-- it has a ton of practice questions and it's a must.
After you've done several practice questions for each question type, take a practice test. Either the Cracking tests (get the version with the DVD) or the official GMATPrep tests
will give you the best idea of your progress. Then you can see where your weaknesses are and come back to ask for suggestions on how to set up your studies. Or just explore the forums...I've found a lot of good info here just browsing around.