It depends on what's in the tank. If you don't have any coral, you can afford to let your nitrates creep up a little, and you should be able to get away with every other week changes. If you have a reef tank, you need to keep an eye on those 'trates, and you might have to change every week.
A good way to figure out if you have to do that is to take a nitrate reading every day, and see when they leave your "comfort zone" for what you'd like in the tank. That's your cue to do the change. Repeat this a couple of times and see if your nitrates rise consistently, ie every 10 days they get too high for comfort. In the long run, this will save you time and money.