The GMAT score will last 5 years from the day you take the test. You should figure out when you want to apply to MBA programs at the latest, or earliest, and plan accordingly.
Studying for GMAT is not like studying for say, economics because when you actually do well in economics class you are going know most of the stuff for a very long time, if not for life. GMAT has many different concepts that you will not see ever again for the rest of your life. Therefore, you definitely do not want to spend time studying and then take the test months later. You will forget a lot of things and your maximum score would suffer.
I think it all breaks down on when you want to apply to schools. Keep in mind that if you get crazy hours when you start working it could be very challenging for you to study.
Hope it helps.
Set your aims high, carpe diem