The frequency of changing the anode rod is largely dependent on the quality of the water source. The higher the mineral content, the more frequently the anode will need changing. But do inspect it at least annually. If your anode rod deteriorates relatively quickly, you might want to check the heater element for encrustation, as well. Keep a spare on hand, but if the element passes conductivity testing and is coated with a crust of mineral deposits, immerse it (don't cover the threaded end and connections) in white vinegar for a couple of days and the crud will come off. It can then be saved as the "spare."