Time to replace the old, dirty air filter in your Subaru? A good rule of thumb is to change it on an interval of every 10,000 to 20,000 miles. Once per year might be easier so you can do it on a regular schedule. Whenever you decide to do it, it's a fairly easy process.
MAPerformance carries many brands for you to choose from. OEM Subaru, K&N Filters, Cobb Tuning and aFe Power are just some of them.