Antananarivy
...it's depend on your basic level of knowledge...minimum you need 4-5 months...
This statement was right at the beginning and wrong at the end.
It absolutely depends on where you start and what score you need. If you're looking at a top15 school, it would serve you well to score above 690. If you fall into a really competitive demographic, such as Indian IT, you will likely need a 730+. That being said, there is no minimum amount of time that you need to study. Take a diagnostic test and see where you stand. If you score a 650+, then a 1-3 month study plan should suffice. I personally wouldn't be able to prep for much more than 3 months without completely burning out.
As for when you should start, I'd recommend that you begin as soon as your schedule allows you to. The test is demanding enough when you aren't in a rush - you certainly don't want to take the test when you're trying complete applications.
Manhattan GMAT's free exam is a good diagnostic tool:
https://www.manhattangmat.com/free-gmat-sample.cfmGood luck!