filters
Yeah, changing the air filters is a in-exact science. I was going to change the engine air filter, a week or two ago, and it looked brand new. I held it up to the sun and I couldn't see any large particulates. I have seen dirty air filters and mine did not meet the criteria. but it was two years old.
It all depends on the area where you are at. You live on a dirt road, you maybe changing every year.
A lot of pollen in your area in the spring, you maybe changing the cabin filter every year. Its not really a you drive X-miles, you change filter hard rule.
If fan-air flow into the cabin seems low unless you are on high fan-speed it may need replacing, just have to check.
Not much of an answer, I know, but I do know if you live on a dirt road, you will be changing it quite often

Take the filters out and look at them. If the cabin filter is hard to get at, then just replace it,
Have a good one!
Parrott
2011 QX56, 20 inch wheels, some options, black on black,