just recently came across a plausible explanation for the expression "for donkey's years", meaning for a very long time
it seems that it is a modification of "donkey's ears" which, as everyone knows, are a lot longer than those of a horse
which, tbh, sounds like a pretty stupid explanation
if they're going to make something up, they could at least make sure that they build an entertaining story around it
hence, being boring, it's probably true