Although my car wasn't built with a cabin filter, it was 24 years old when I got it, and by that time nature had provided one in the form of packed leaves, pinestraw, and acorns. It was like a miniature fireplace in there, with that nice hot blower resistor ready to set them all off.
To the past owners' credit, at least one of them had taken the time to install a fire extinguisher, so I guess he was ready for the big day.
I should probably look at that again when the weather improves. Assuming I do, I guess I can say I clean the "filter" every 4 years.
Truthfully, I never knew modern cars had filters for this until pretty recently. So you don't even get to smell the dead skunks and cow manure anymore? Good times man.