Ash has a lot of BTUs and can last a good while, but it will not if your stove does not regulate the burn. Other wood, like Black Cherry (which has a lower BTU rating) seem to hold coals better in those stoves.
My old stove in the old cabin loved Ash, and it lasted a good while, but the flue on that did not draft real well. In the new cabin, the flue is much improved, and the Ash does not last nearly as long.
In the old stove I used Ash all the time. In the new stove I use Ash during the day, and Cherry or Hard Maple at night.