You know when we say 'donkeys years', where does it come from? We might say 'oh I've not seen so & so for donkeys years or I've not done such & such a thing for 'donkeys years.
An exceptionally long period of time. The phrase likely originated as rhyming slang, where "ears" rhymes with "years" and alludes to the length of a donkey's ears. Throughout time, it became more popularized as "donkey's years."