Every week, if not twice a week, no one realy dose though.
The best advice is to test as often as possible, but as you go along you get a feel for your water just by the look and smell, and by watching your fish.
I keep discus, and i only test the water during a water change to make sure its the right PH etc before i add it.
If you are just starting up the tank, and have not previously cycled it, then I recomend you test it daily, because you will have massive ammonia and nitrite build-up durring the cycling period.
Other than that, i periodically check my levels, and also make sure i test them when I am having problems, such as illness, after a filter cleaning, or after a power outtage.
Hopefully it is cycled, so if it is, check levels about twice a week if it is less than a month old. After a month or two, depending on how stable the levels have been, you can just go once a week when you do water changes and when adding new fish.