It really depends on what you're trying to get out of it. Answer these questions and it will help us answer your question better:
What school(s) do you want to attend?
What score are you aiming for?
What upper level math classes did you take in college (i.e., calculus, algebra, etc)?
What grade did you earn in each of those classes?
Are you a native English speaker?
How much writing do you do?
Is you current job very technical where you have to read lots of data and analyze it?
How many books/magazines do you regularly read in a month?
Did you take any philosophy courses in college to help with critical reasoning?
Do you know how to diagram a sentence, or do you know what this means?
Are you currently employed?
If so, how many hours per week do you work?
How much travel time is requried for your job?
In my opinion, the GMAT does a good job at testing the various skills necessary to succeed in b-school. (Or at least is appears to since I am not yet in b-school.) You can be the best analytical thinker but if you have no skills at verbalizing your thoughts or results to others, no one is going to think you're very smart.
The verbal part of the exam is the most important part of the exam. It can make or break your exam. Sure, the math is probably easier to improve upon, but the verbal seems to really be a hurdle for people to get over. It may not be true, but the verbal seems to be weighted heavier than the math. I scored an 84% in the verbal my first take and got a 82% overall. I scored 95% in verbal second time around and got a 95% overall. You might find similar results for others as well. Think about the questions I asked, answer them and I'll reply with my estimate of how long you should plan to prepare.
I prepared for 3 months part-time and went from a 650 to 720. I spent about 20 hours a week at it.
Im planning to take the GMAT, how long does it usually take to prepare for this exam?
J Allen Morris
**I'm pretty sure I'm right, but then again, I'm just a guy with his head up his a$$.