if the tank is established and the filters have cycled, then check the water quality once a week before you do a water change (as suggested by Gilli). The PH can be checked several times a week and if it is stabile then you can check it once a week.
If you lose a fish, the water goes cloudy or starts to smell, then test the water.
Before you go to the LFS to buy any new fish, test the water.