Forrest fires can cover an entire coast line and Amaterasu won't stop burning until everything it's in contact with is dust.
The flame remains for 7 days and 7 nights, those are DB and manga facts, that's long enough to cover a portion of a region.
No element can stop it proven by it's ability to be used on water and burn fire, it's like TSB in the sense it can't be negated, but unlike TSB it doesn't have a weakness to Senjutsu, it's only weakness is the user.
Also you can't compare the two attacks they work differently, the only thing we know is BZ witnessed RSM, TSB, Rinnegan and knew all of Kaguya's feats yet still considered the black flames to be the strongest physical attack.
The Amaterasu we've seen is the restricted form up to this day.