K&N’s replacement air filters are designed to increase horsepower and acceleration while providing excellent filtration and increased fuel economy.

Subaru Service Interval: Every 30,000 miles. 15,000 if in harsh dusty enviroments.

Subaru recommends replacing your air filter every 30,000 miles or sooner if exposed to dusty off road conditions. So if you've been rallying in the woods RalliTEK suggests every 15,000 miles. 

Here is a little info on K&N filters:

Making over 1,200 different replacement air filters for virtually every vehicle on the road. These filters are washable and reusable and are custom designed to fit into your existing factory air box. Their drop-in design means adding performance is simple. With most vehicles you simply remove the disposable air filter and replace it with a K&N. 

K&N stock replacement air filters for street vehicles are covered by the famous K&N Million-Mile Limited Warranty™ and will be THE LAST AIR FILTER YOUR VEHICLE WILL EVER NEED.

RalliTEK's thoughts:

Inspect it and clean when necessary every 15,000 to 30,000 miles...