Being in a box makes no difference - the standard air filter is in a box and that gets changed yearly. You need to clean your air filter once a year along with your standard service whether its open air, in a box, or bolted to the bumper!

being in a box, the air coming in is still dirty and getting filtered, therefore the filter is becoming dirty

as Nick says, it needs to be cleaned still

K&N cleaned! 10k miles in 12 months means for a dirty filter!

K&N also say that these are good for Upto 50k miles between servicing.

Before;



During;



After;



And back home;



1st bucket:




2nd bucket:


3rd bucket:


1st and 2nd bucket used a tap, forcing water from the inside out through the filter

3rd bucket was a slim shower head inside the cone!

Top tip, use an old bucket and add some fairy into the bucket to stop the oil trying to cling to the side

Ill check mine again but wasnt half as dirty as that last i had her out

Gets worse during the winter, I always clean mine every 10k miles

That's one clean throttle body!

Urghhhh utter filth that was!!!