A water heater is not usually replaced until it leaks. Heating controls of both electric and gas water heaters can be replaced, if needed, without replacing the water heater when you have no hot water and the tank is not leaking. If the tank leaks, the water heater must be replaced. How long a water heater last depends on how often the water heater heats water and the quality of water being heating. The steel tank of conventional storage tank type gas & electric water heaters expand when water is heated and contract as water cools inside the tank. Also low PH of water and minerals in the water effect the lifespan of the steel tank. With this being said, the answer is unknown! In most cases, residential gas and electric water heaters have a 6 year tank warranty and 1 year parts warranty from the water heater manufacturer. Commercial water heaters nominally have a 3 year tank warranty and 1 year parts warranty from the water heater manufacturer.