Hello, How dirty they get is a function of how dirty their environment is.
I believe big dirt knocks off a dirty filter and more can be blown or washed out but the wee little bits are deep in the fibers and will not come out for love nor money.
Here you might coax 1,000 or 2,000 miles out of an old filter. Not really worth it.
My favorite point is that the layer of filtered dirt can take on moisture and smell. Any charcoal is not designed to combat that effect.
In my automotive setting it's looking like 25,000 miles is a good, average cabin air filter change interval (CAFCI-?). Kira