We have wind, dust, a dog and rain here. I try to remember to clean the filters every month since we run the stern ac almost daily in the aft. Thinking I should clean every two weeks. Have a nice, old soft bristle toothbrush that works fine.
Most can not go a month on those small filters without cleaning them.
And remember, the evaporator likely needs cleaning as well-- certainly once a year.
The filters on all the RV roof airs are less than effective!