I think there's lots of variables.
One variable is the variety of wood. For me, cherry, beech and apple seem to dry pretty quickly, whereas oak will take longer. I think it's because of the density of the wood grain, but I'm not sure.
If it's whole rounds, they'll take longer to dry than splits.
Also, depending on how it's stacked. If it's stacked and covered, it'll take longer. If it's stacked and only the top covered, but the air is allowed to blow through the stack, it'll dry quicker. If it's stacked in a constantly shady spot (like in the woods) then it'll take longer to dry than if it's in a good warm, sunny spot.
In short, I've had cherry and apple be nice and dry in only a couple of months, but I've had some fresh cut oak take twice that long, if not longer.
I also know of some who purposely use green wood for smoke too.