I stumbled across a video on how to change the cabin air filters on our cars, and decided on a whim to check mine. I didn't expect to see much, since it is not particularly dusty around here and the car is not old. At about a year and a half and 45,000 miles on my car, here is what I pulled out!
Yuck! Way worse than I thought it was going to be.
Keep in mind that these are not HEPA-type micro filters. They just filter out "the big stuff", and can be cleaned/reused if you are careful. I blew all of the debris out with compressed air first, used the sprayer on my kitchen sink to give it a high-pressure rinse, press-dried it with a towel, and used a hairdryer to finish the job. Don't worry if it is still discolored afterwards as that won't affect it's function.
As an aside, it is NOT necessary to empty your glovebox to do this. The glovebox is removed by pinching the sides of the box inward to release two tabs. This will allow the glovebox to fall down, but instead of letting it drop you can just pop the two lower pivots for it off by gently pulling the whole glovebox upward and outward. There are two pivots, one on each side, and they just clip on. Then you can just set your glovebox...with contents...aside.