The '03 is now at 57K, blown out with compressed air periodically and inspected. It has never been replaced. Just coming up to a plug change after 20K. I will probably check the air filter again at this time. With the Renntec crash bar in place in place, the filter can barely squeeze past it's housing.
I checked mine for the first time since the spring. Oops, I should have checked it sooner. It is rather filthy after the 12,000 km that I have put on so far since then. If it were a car filter it would be replaced. I am going to try blowing it out with a compressor to see how clean it can get.