Welcome to PrepAdviser and thank you for your question.
The GMAT score is valid for five years.
My recommendation is to take the GMAT before you start shortlisting the business schools to which you will apply. Once you know your GMAT score, you can make a really good school selection. To improve your chances of admission aim at schools where your GMAT score will be above the average of the last incoming class. You can also decide to retake the test and achieve a higher score before you apply.
Let me know if you need further details.
Iliana Bobova Head of Admissions Consulting PrepAdviser