Hi leminhyen2,
The Official GMAT Score is valid for 5 years, so you should plan to take the GMAT within 5 years of when you plan to apply to Business School. Many Business School Programs expect applicants to have at least 2-5 years of quality Work Experience though. While there are some exceptions (certain Schools will consider admitting strong applicants who don't have little-to-no Work Experience), you have to account for the time that it will take you to acquire that Work Experience into your overall application plans.
If you're just beginning your undergraduate university studies, then it's probably too early to be thinking about the GMAT.
1) When do you think that you'll apply to Business School?
GMAT assassins aren't born, they're made,
Rich