Put a restriction gauge on it (Filterminder, Informer, etc.) and leave it alone until it shows significant restriction. No reason to touch it any more often. Look at the link below to see how and why.
Air Filters and Economy
This 2009 report tests the effect of dirty air filter on fuel economy using standard EPA dyno routines. They used later model EFI cars (2003-2007)and one 1975 carbureted car. The conclusion was that the fuel injection could compensate for restriction up to the point where the car's performance had deteriorated to being barely running and fuel economy decreased only by a percent or so maximum. The carbureted car was more effected but fuel economy wasn't lowered by nearly as much as "common knowledge" dictates... only 2.5 percent.
In every case lower end performance was unaffected until the restriction reached the level at which insufficient air was being inhaled. Performance dropped mightily but the EFI could trim fuel to keep the mixture about right. They found that in "normal" driving situation, the car was almost unaffected by a clogged filter because the airflow needs are so low in most situations. In a few cases, the filters were so clogged that the engine sucked them into the intake tube.
This research also reinforces the fact that a "free-flow" air filter will not contribute much (if anything) to fuel economy. Some gains might be noted if the entire system was changed and a tuned intake added, vs just a replacement filter element.
Power output is a separate matter, but these days, the OEs are giving cars decent systems with more than enough flow and even some good tuning. But, like everything else, how much you gain by replacing a system is relative to how good the new is vs the old and whether it meets the engines flow requirements.T
