In my neck of the woods, air filters need to be replaced only once every
> two years or so; they don't get dirty enough to warrant replacement
>before then. I had replaced mine in Jan 2005 and wasn't due for a
>replacement for another 8 months or so.
I think it has to do with placement of the air intake on newer cars.
When they had the air-intake horn under the hood, I could change a dirty air filter every 4 . 6 months.
With my 2002, the air feed is from somewhere under the fender. After a year, the air filter looks like new....