Extract from Rheem Owner's Guide
About your Water Heater
How long will the water heater last?
There are a number of factors that will affect the length of service the water heater will provide. These include the water quality, the water pressure and temperature (inlet and outlet) and the water usage pattern. However, your Rheem water heater is supported by a comprehensive warranty.
An anode is installed in your water heater and will slowly dissipate whilst protecting the cylinder. The life of the water heater cylinder will be extended by arranging for an authorised person to inspect the anode and replace it if required.
The suggested time after installation when the anode should be inspected is:
Rheemglas 8 years
Optima 10 years
For softened water supplies or in areas of bad water quality, it is recommended the anode be inspected 3 years earlier than shown.