At Donkey's Year's Meaning?

chiefly British. : a very long time.

Is donkey's years an idiom?

If something lasts or has been happening for donkey's years, it lasts or has been happening for a very long time. I've been a vegetarian for donkey's years. He owns some old iron mines that haven't been used in donkey's years. Note: This expression was originally `as long as donkey's ears', which are very long.

How many years is donkey's years?

The OED defines “donkey's years” (also “donkeys' years”) as a colloquial usage meaning a very long time. It describes the phrase as a “punning allusion to the length of a donkey's ears and to the vulgar pronunciation of ears as years.”