When should I replace my air conditioner? An average lifespan of an air conditioner is 10-15 years. When your unit gets older it can cost you more money because it’s not running efficiently. What should I keep my thermostat at? You should keep your thermostat at the same temperature you are comfortable with so that you don’t have your unit working so hard going back and forth on cooling. How often should I change my air filter? In Arizona depending on which air filter you are using anywhere from 30-90 days.