My biggest engines are P-120's, so it may be different for the larger motors, but I have UATs that have been in service for 4 or 5 years without changing the filter - which I usually do when I take the UAT out and re-build it for a new airplane. As long as your fuel is effectively filtered before going into the aircraft and nothing is "growing" inside your aircraft, I have yet to have a UAT filter get full enough to effect fuel draw on a 120 or smaller sized motor...

For a P-200, it may be more of an issue - requiring more regular replacement...

Bob