Donkey's years

This one is a pun stacked on a pun. It plays on "donkey's ears," which are famously long, and relies on the fact that in British non-rhotic pronunciation "ears" and "years" sound close enough to swap. It's reinforced by the folk belief that donkeys are unusually long-lived animals. The wordplay is recorded in print from the 1910s, and the spelling shifted from "ears" to "years" early on, cementing the pun as the phrase's permanent form.