There is no real answer to the question unfortunately. A lot of people who end up with 700+ could probably walk in cold turkey and take the test and get a 400 easily. A person who speaks english as a second language and is weak in math would probably have to study pretty hard. Best advice, go download the free software off mba.com and take the first test and see how you do. If you score a 600 or something just take the test if you are borderline 400 go buy the official GMAT guide.
Read the front section, the math, and the verbal part. Do a bunch of the questions out of the book. Then take the of the online practice tests from mba.com and see if you are ready.
Make sure you at least go onto MBA.com and read about taking the test. Amazingly people are suprised sometimes to learn things like they can't use a calculator and that its totally computerized. Learn the time limits and what the sections are. Remember its an expensive test and you can't take it for a month after you took it so don't rush out to take it if you dont feel prepared.