In donkey's years means in a very long time indeed. Not seen him in donkey's years.
It is used only in the negative, about a gap rather than a duration. The phrase probably plays on donkey's ears being long, and donkeys do live a long time.
1phrasein donkey's yearsIn a very long time indeed.
“I have not been down that road in donkey's years.”
in donkey's years is a phrase.
Practised in Idioms