To keep your engine running efficiently and improve fuel economy, you should check and/or change your Air Filter every 6 months. The way every car works is different, so it is hard to give a specific mileage figure or timeline of the life of your cars filter.
A filter lasting 20,000 or even 30,000 miles on a vehicle driving mostly on expressways may only last a month or two in a rural setting, since terrain is different and you’ll likely be driving on gravel roads.
A good recommendation for the city driver may be changing it annually/every 15,000 miles for preventative maintenance, but the same will not be said for the typical country driver.
No matter what the mileage or time, a filter needs to be replaced before the point where it creates a restriction of airflow that is significant. When exactly the point is where that is reached is subject to professional opinion.
Debris trapped by the filter element helps screen out smaller particles that are trying to get through. But eventually every filter comes to the point where enough pressure has built up and restricts the airflow.
Emissions, fuel economy and performance begin to get progressively worse and deteriorate until the dirty filter is replaced. Generally, with heavy duty trucks, when the filter becomes dirty the “restriction” meter on the air filter housing will set a code and display a message that the air filter needs replacing. When lacking such a device, the best you can do is guess.
If you remove your filter and hold it up to the light this should show you how dirty it is.
Excessive dirt, build up, debris and/or moldy discoloration are all indicators that the filter’s properties are compromised, and replacement is highly recommended.
Note: Often there are filters that appear to be dirty but are in fact still good and do not really need to be replaced. Whether or not you replace it is up to you based on the recommendation of your technician.