Though there is no set rule, generally, a quality heater can work well for up to 2 years and sometimes even longer. If it is working perfectly, there is no need to change or replace it. If any part is damaged, especially the thermometer, which gives wrong readings, then it is time to replace the heater. You must carry out regular checks to ensure that the heater is working effectively. Any kind of problem can lead to a very bad effect on the fish. If you find the temperature fluctuating too much and too often, you must change it immediately. If you are careful while using and regularly maintaining it, you will not have to replace the heater very often.