The phrase 'donkey's years' is commonly used to indicate a very long time, and is mostly used in a sentence thusly: "I haven't seen you in donkey's years" or "It's been donkey's years since I had one of those!" The phrase can also be used more abstractly, such as "Old Aunt Maria? She must be as old as a donkey now!" though relating dear Aunt Maria to a donkey may not be acceptable in all situations.
Donkeys live for practically ever. That is why this phrase came about. The oldest live over 60, with Lively Laddie taking the prize with 62 years to his name.
Because of this fact, donkeys are very useful working animals. They can still breed past ages of 30, though you'd be probably be pushing it once they're much older than 35. You can start training them before they are five, and you can basically work them until they are dead, though as a nice kind donkey owner you should ease them into retirement as their joints get stiffer.
The phrase 'Donkey's Years' probably originated in Britain, but these days it is used by people all over the world.
Donkeys' Years is also a play. It was written by Michael Frayn and first viewed in 1976 at the Globe Theatre in London. The play is about six former students who go to the 25th anniversary reunion at the college they attended.