Most manufacturers recommend cleaning cartridges when the filter pressure rises 8-10 pounds above the start up/clean pressure. However, some filter's pressure won't go up even when the filter elements are filthy so... most pool professionals in Arizona recommend manually cleaning cartridge filters once every six months. After a few cleanings, you'll learn more about the pool and how dirty the filter is becoming and can vary the cleaning intervals.
Here are a few pictures from a recent filter cleaning job completed in Mesa, Arizona.
The manual cleaning process takes somewhere between an hour and two hours depending on how dirty the filter is, how many elements are being cleaned, and how large the elements are.
Thanks to Mesa Pool Services for the pictures from their recent job!