They say you should allow a year per inch of thickness for air drying wood and that this doesn't hold true for particularly thick pieces either so a year for your piece was no where near enough.
What most people do is to rough turn to reduce the thickness of what needs to be dried, allow it to dry knowing the wood will go out of shape and then re-turn to the final thickness. this way, you can generally reduce drying time to a few months.
In answer to your question about when is the wood dry - the answer is never! It equalises to it's environment and you can figure out when that has happened by weighing the piece of wood and when it stops changing weight, it has equalised. Move the wood to a different (wetter or drier) environment and the wood will either absorb or lose moisture, which will result in a change in weight and possibly a change in shape too.