DIA and IAD, at least, will let you park your car for longer AS LONG AS you let them know that you'll be parking there longer than their posted "limit." Dulles doesn't have a max limit but says vehicles left longer than 45 days will be declared "abandoned" and removed at owners expense. But if you register your car with their parking office, they won't declare your car abandoned.

So think most airports just assume (perhaps correctly) that any vehicles left over 30 or 45 days are abandoned and they don't want them taking up space.