I echo Farmer Steve, "... it will depend on species as to how dry it is if it is standing" but I will try to provide a little more detail. Of course it also depends on the climate and micro-climate where you live, the predominance of wood boring insects and fungus, and the reason it died. If it died of Heart Rot, it will rot from both outside in and inside out, quickly becoming worthless. Down here in our 90% humidity and No Winter if it hits the ground you better pick it up in a month or two or the termites, Long Horn Beetles, mushrooms, and fungus have got it. Dead standing pine will last a year as long as it is Standing, but even then I have seen the first couple of feet of sapwood rendered useless by termites. I have also seen Standing Southern Pines 40 feet tall that died in a fire but had a lot of heart wood pitch ("Southern Fat Pine") ten years old and still Very Solid! Can't burn it in your wood stove because of the creosote, but makes good starter and kindling and a Wonderful Bonfire!
Hickory and Pecan are good for a year standing down here, probably much longer where you are. Live Oak will last Many Years standing (I recently harvested some that was dead standing for 10 years and dry and hard as a rock. Difficult to cut and split, but Very High BTU!) Red Oak rots very quickly, in fact it will rot even standing faster than it will dry out, and forget it if it is on the ground for 3 months in our swamp. White Oak is interesting. The sapwood rots very quickly, and will in fact rot completely off the tree while it is still standing, but the heart wood is resistant to rot (high in tannins and very dense) as long as it does not have Heart Rot. I just harvested 2 white oaks standing dead for 2 years. The sapwood was rotted completely off. The heart wood was still wet, heavy, and Very Hard!
I harvest a Lot of Dead and Dead and Down. I stab it with my knife and I can generally tell by how fast and how deep the knife goes in how solid the wood is. A portable drill and a long reach half inch bit will give you Much More Detail, simply watch the penetration rate and examine the shavings as the bit drills deeper. A nice thing about Dead and Dead and Down is that even though the heart wood may look like it is just as wet as the Green heart the dead heart will still dry out much faster after it is split than green wood. If I split green wood I have to let it dry at least a year before it generates very much heat. If I split a dead and down or dead standing in August it will generally burn pretty well by March, even though in August the heart wood Looks wet and the splitter squeezes water out as it splits.
I fully agree with Marine5068. "Seasoned Standing" is a rare thing indeed down here. In fact, I have never seen any, but then we don't have locust or very much ash. Anything much over 4" diameter down here MUST be split before it will "Season".