How many years should you change out your hot water heater?

Every 3 months would be fine. I change mine once every 2 months just to be on the safe side.

Actually some change it more often as the water is such poor quality that they lease the heater rather then buy one so they take a 3 week lease with an option to upgrade as needed

If you are talking about a 30/50 gal residential hot water tank, they may have to be changed every 10/15 years. If you are not getting proper regulated hot water in the shower, then it might be time to change the tank.