I've looked at my air filter a couple times in the last 3 months, and I thought it looked really clean. Decided to change it the other day since it has been a year. When I placed it beside the new one, I realized just how dirty it was.
The sunlight was basically the same on both, but the clean one is reflecting so much more light because it is so much whiter/cleaner. I don't recall ever being on anything but paved roads. I think the moral of this story is don't try to determine how dirty the filter is by just looking at it, but change it routinely for best protection and performance.