I've got about 9 cords split and stacked this year .Probably 3 of that is dead ash either standing or wind blown under 12" in diameter .Half that was under 6'"and didn't need split. All that little wood was cut with an 024 Stihl,light and fast enough for that type stuff .
Usually on that stuff the roots fail then the wind gets it ,maybe 4- 6 years dead .It takes a long time for ash to get punky laying down .
I have no idea how long a large ash will stand .Eventually the wind gets the upper limbs first and I suppose in time will get the whole tree given enough time .
In the last several years I've saved the larger stuff for lumber logs and have 8 to 10 thousand BDF stacked up .Most are a tad over 16 feet and between 20 and 36" in diameter .Some of those were around 50 feet up to the first limb ,nice stuff .
You can always make firewood out of lumber logs .Once it's firewood you can't go the other way .