Many people who are considering applying to Business School in the future want to know when they should take the GMAT.
The first thing to know is that your GMAT Score is good for 5 years. This means that you should feel free to prepare for the GMAT and take it when you have some time, NOT right before you're gearing up to apply. Indeed, many people would be very well-served taking the test during their senior year in college, as they're still in an academic, test-taking mode at this point (and still vaguely remember, say, the formula for the area of a trapezoid).
At this point, you're likely reading this blog posting thinking Great, where were you 4 years ago when I was still in college? Don't worry about it, as the average GMAT test-taker is in his or her mid-to-late 20s (70% from 22 -30).
The general principle is that you are best served taking the test when you have the bandwidth to fit it into your schedule. Ideally, you'll have your GMAT Score established before you gear up to complete your applications. Indeed, your score may even change which MBA programs you apply to (for better or worse). When app deadlines roll around, you'll want to be worrying about honing your essays and arranging school visits - the last thing you want to have in mind at that point is getting a higher GMAT Score.
Another reason to take the GMAT early is that, if you're determined to get a certain goal score, you'll have a much better chance of doing so if you give yourself a runway. The correlation between time spent studying and one's score is consistently positive - the average person spends 2 - 3 months preparing for the GMAT. Indeed, the average score increase for someone who takes the test a 2nd time is 31 points (according to data from GMAC). As you can only take the GMAT once per month, you'd like to give yourself time for multiple tries if necessary before any deadline creeps up.
Now you may be thinking, Wait a minute, I can take this test more than once? That's right, Business Schools take your best score, not your average score. So there's really not much of a disincentive to take the GMAT multiple times, aside from your time (and the $250 per sitting it will cost you). This has its limits though - Harvard Business School, for example, told us that by the 5th try, they tend to discount your score a little bit. So try and get your best score possible within your 1st four tries or so.
Bottom line - wait until you have some time on your hands, but generally the sooner you prepare and get your test score out of the way, the better off you'll be.