The bad thing about kiln dried lumber is it doesn't want to stay that way. If it isn't stored where moisture content of the air is in the 5-8% range, the kiln dried quickly picks up the moisture out of the surrounding air (usually 10-15%) and looses the benefits of being kiln dried. The rule of thumb for green, fresh cut lumber to dry down to as low a moisture content as the surrounding air, is one year per each inch of thickness. A 2X4 should be dried for two years before you consider it dried. Length and width doesn't come into account. They use to recommend Lead base paint or wax on the end grain to keep moisture from escaping through the end grain and causing those long splits at the ends of the boards. You want the moisture to escape through the sides, top and bottom of the boards, that is why it takes so long to correctly air dry lumber. Once it is air dried like that it is about as close in moisture content as kiln dried lumber that has been stored out doors under a shed . They all seek to find the level of the air that surrounds them.