Donkey’s years OR Donkey years

 “Donkey’s years” is preferred. It is an expression that means a very long time.

The phrase is believed to have its roots in rhyming slang specifically from the phrase “donkey’s ears” which rhymes with “years.” Over time, “donkey’s ears” morphed into “donkey’s years,” becoming a popular idiom for describing a very long time.

The possessive form “donkey’s” is preferred over “donkey” because it follows the original rhyming slang structure of the expression.