donkey’s years

A long time. The origin here is disputed. Some say it is a rhyming  term  for  donkey’s  ears,  which  are  quite  long, and  possibly  also  a punning  allusion  to  the  Cockney  pronunciation  of “years” as “ears”; others believe  it  alludes  to  donkeys  being  quite  long-lived. The  expression  dates only from the late nineteenth century. Edward Lucas used it in The Vermilion Box (1916): “Now for my first bath for what the men call ‘donkey’s years,’ meaning years and years.”