Ive had the car less than a year, but have now marked when I last changed it and the milage. Something that I found curious is that I also changed the engine air filter today and that wasn't as dirty as the cabin filter. My only guess is they (prior owner) didn't know there was a cabin filter in such a tiny car and never changed it, or on a random oil change the tech may have asked if they wanted to replace the intake filter, so that could also be why the intake filter wasn't as dirty. Dunno, but they both are new now.
I have a new cabin filter in stock for when needed. Checked mine after a year and it still looks brand new. Will check it at the two year mark this summer, but I don't expect to see much difference then either. Rarely use the AC and have only turned the heater on to see if it works.