BobP times two. Wood does some strange things. They used to say for air drying lumber that it took one year per one inch of thickness to completely air dry lumber. That would be two years for a 2x4. Then it would only dry it down to about 13% moisture content where kiln drying takes it down to around 5-6 %. If you let that 6% kiln dryed piece of wood lay around exposed to outside air then it will quickly pick moisture from the air and climb back up to what ever the moisture content of the air is in your region.
Side Bar- Lumber is cut fairly straight and true. Crooks, twist, bows, cups are the result of removing moisture from the lumber. This allows internal stresses on the cell walls to deform the board. Take a board with 100% moisture content, meaning it can't hold any more moisture, and start drying it. Down to about 29 % it only looses moisture in the cell cavity and no shrinking takes place. From approx. 29% on down to about 6% is where the board starts loosing moisture in the cell WALLS and this causes deformation, twist etc.
Sorry for the dry lecture, I slipped back into the classroom for a moment. Musky Glenn