The Audi Cabin Air Filters are a new addition to what engineers are placing on their vehicles to help clean up the air the passengers of the vehicle breathe. Before the use of the cabin air filters on your Audi, there was only a large hole with a screen on it to stop large debris from entering the system.
The location of the Audi cabin air filter is at the air intake portal just below the windshield wipers at the bottom of the windshield on the outside of the vehicle. To locate them the hood of the vehicle will have to be raised. The cabin air filter on most vehicles for this make is HEPA filters in their quality. That is to say they trap up to 99.7% of all particulates of 5 microns or larger. This is the same quality of air that hospitals use.
The replacement of the cabin air filter on your Audi is really determined by the quality of air the vehicle is exposed to on a constant basis. If the air is filled with a high number of small particulates like in the city or in a dust or dirt road, the changing of this filter element will be more frequent than if the Audi is used mainly on the highway.
Depending on the model vehicle you have the cabin air filter is held in place by a cover that can be removed with a screw driver or loosening of the clamps. An inspection of this component should occur on a monthly basis with replacement annually or bi-annually by the owner of the vehicle. This will keep the air quality in the cabin of the vehicle to a level expected by the owner.