The best time to give the GMAT test at least 6 months before the MBA batch starts. If you are planning for fall 2016 then you should take the GMAT in February/March- 2016. Most US universities have 3 deadlines. Round-1 starts in August/September, Round-2 in November/December and Round-3 in March/April, but international students don’t have a great chance of getting admission in the last round. Applying in the first or second round gives you a good chance of getting admission as well as a financial aid or scholarships.
If you give your GMAT 6 months before you apply, you also get a few more chances to reappear for the GMAT, if you are not satisfied with your previous score. If you are going to be a graduate soon, then you should take the test soon because with a fresh, mind you will be able to solve the maths with a good speed.