I think it all depends on the environment.....obviously, driving in dusty, dirty, muddy areas will require replacement more often.
As far as modified house hold filters for use in the cabin air filter slot, sure, go ahead if you are in a pimch; it's better than nothing; but the modern/regular CAF's have other materials within to aide in filtering, as mentioned, activated charcoal, or in the case of the FRAM "Fresh Breeze" they have baking soda within too, to help freshen odors as they pass through the filter.
Something I've learned, if you ride with the windows down a lot, be sure the "FAN" and AC switch is Off too. No point in "filtering" the outside air with your windows open...now, if you got the windows rolled up, go ahead and adjust the ac/fan as appropriate.