What is the average lifespan for a water heater in Phoenix, Az

That depends on several factors as listed below:

1) Amount of minerals in the water, for the most part Phoenix has lots of minerals in the water, meaning a large amount of mineral content (ppm) are present.

2) Flushing the tank once every 6 months when minerals start to collect in the bottom of your water heater tank, it's first actually like in a jelly form, before they solidifies (harden), this process takes about a year or so, that's why you want to flush your water heater tank under pressure, about every 6-months to a year before the solidification process takes hold. Remember, you do NOT want to drain the tank with the water inlet valve turned off.

3) Change the anode rod, the anode rod should be replaced when it is 50% spent, or at the least every 3 to 4 years.

If the above maintenance is done on a regular schedule, your water heater should last an incredible 15 to 25 years, if not, it will only last 7 to 10 years on the average in the Phoenix and most surrounding areas of Maricopa county.