air filter longevity

[Mercedes Fred]

Ive an '88 300e and after about a year, the air filter is still reasonably clean...what is the life on those babies? i highway drive daily in the country, small community (no industrial air pollution), no traffic jams).

There a small hole at the top of the air filter container which is open and has a rubber gasket on it; it does not appear to vent into the fuel injector; is this supposed to have a hose connected to it? Ive never seen a new 300e engine, so iom unsure of there is something missing here that allows the air to bypass the air filter.