I wander if my grandpa fisher is an epa model???? Lol sure cranks out the heat either way!It depends... mostly I try to stay at least a year ahead. But I have oak that has been stacked for going on 3 years. Black locust I've had even longer (that's my Armageddon stash.) Dead standing/down ash can usually be burned right away. Some of the uglies get burned before their prime just to get rid of the stuff that doesn't stack well.
My OWB is not an *ahem* EPA model so it's very tolerant of wood that is not primo. But I'm not like some guys that drop a mountain of green rounds in a pile next to the OWB in the fall and burn through it all winter. I like more heat and less smoke from the wood I burn.