Keep in mind that a dirty air filter will filter out more dirt than a clean air filter.
Subaru (IIRC) recommends 30k between air filter changes.
You can't hurt anything by not changing the air filter soon enough or too often, although performance and gas mileage may slightly degrade if it really gets clogged.
Someone out there sells a gauge which you can install into the airbox which measures the pressure drop after the filter vs atmo pressure and lets you know when the filter really gets clogged. This is the best way to know when it's time to change the filter. Filter Minder
I've been thinking about trying one myself, but not sure where to buy one.