This all depends upon you.
Go to mba.com get familiar with the Gmat structure.
Then download gmatprep from mba.com and give one exam on the stretch.
After the test analyze your performance on both quant and verbal. Note down the areas in which you are strong and in which you are weak.
You should have a target, which might depend upon the level of school i.e. 600+ or 650+ or 700+. Then start working on your weak areas and nourishing your strong ones.
Now it all depends upon you to fill the gap of present stage and the target one. Some have strong verbal but poor quant and vice versa. So the total time only depends upon YOU....so before thinking of anything else:
1. go to mba.com and download gmatprep.
2. give the exam.
3. note down your weak/strong areas.
4. decide your schools( gmat score you looking for)
5. come back here mods/admins can better suggest you further.