I replaced mine at 30k miles and it was pretty disgusting. There were leaves and all sorts of crap in there. Only reason I changed it was because when you switch the ventilation from outside to recirculating, you can hear the leaves stuck in there. I'm most likely going to change mine at least once a year. I bought my filter off eBay for like $12-$14 (i forget), but it looks identical to the subaru oem one ($30 ripoff).