Best to split when the tree is dropped -- full rounds won't really dry; they'll rot eventually. Stack off the ground, and cover, but try to allow air circulation -- don't swaddle it tight. I sometimes fold my cover up in the summer for better air circulation, but leave the sides loosely covered in the winter to keep snow out. 1-2 years to fully dry depending on the condition of the tree and time of year when it was felled. If it's split smaller, it'll dry faster.

Wood that's been around too long will feel soft -- you'll come to recognize is pretty quickly. If it's not too far gone, I will mix a little in to an established fire, but if it makes a lot of smoke, you don't want to do that unless you're burning in a fire pit outdoors.