Depends on the media. Some media needs to be changed quite often because it gets clogged and can't be cleaned. Some brands of filter element will get matted with attempts at cleaning, reducing water flow through them. Sponge inserts will last indefinitely and can be cleaned often to keep them clear of solid material. I have Aqua Clear foam inserts that are over 10 years old.
Regardless, the inserts need to be cleaned as needed to work properly. Nitrifying bacter are aerobic, and if the filter material gets clogged and anaerobic conditions are created they will die. Nitrifying bacteria don't consume solids, so they have to compete for available oxygen with the bacteria that do. Keeping the filters cleared of solid material will help keep nitrate levels lower.
So, the inserts should be cleaned as needed (in tank water), and this depends on the individual tank as to how often is a minimum. Cleaning inserts before they get plugged up is good practice, and cleaning too often does not exist in an established filter.
