When I have bought air-dried wood, it was dried for 2 years.
I think if you put 20,000 bf of wood up, and plan on selling it for $3/bf, most guys don't want to wait 2 years for that money to roll in. Hence the kilns.
There is also the possibility of a solar kiln. I think the deal here is you keep it in a dry, protected, well ventilated area, but with the south side glassed in.
The other disadvantage of air drying, which was explained to me, is the rot factor. Seems the softer woods such as basswood will get a bit moldy during the natural air drying process.