This greatly depends on the type of water heater you have and the brand. We suggest that you start inspecting the performance of your heating unit as soon as 6 years. This does not mean that you will need to replace the unit after 6 years. But you will need to monitor performance and keep track of water heater ratings. You could just as well have a 20 year old unit that works flawlessly. The quality of the water will play an important role when it comes to the life expectancy of your water heater. Consult our best whole house water filter article to learn how to prolong your water heaters life.