Air Filter Replacement Time?
 

So how long between air filter replacements?
Mechanic today said when looks dirty.
I mentioned and he replaced it and service history shows it's been 6 years!!!!

Good question and one I will watch!

I have a second car which is 6 years only but only had 9k on it when I bought it.

Full service history but from what I can see basically oil and filter each year.

Volvo will say 36,000 or every 2 years (as an example) but what happens if you have done 2 years and 3,600 miles, does it still need to be replaced?

36,000mls or 3 years whichever comes sooner. Same as the spark plugs. I meticulously maintain my cars, but only change the filter when it begins to look dirty. Why change a clean filter for another clean one?