What is the origin of the idiom "donkey's years" and when to use it?

The exact origin of the idiom "donkey's years" is unclear, as it has been in use for quite some time. However, it is believed to have originated in British English during the early 20th century. It is often employed in informal conversations or written expressions to emphasize the duration since a particular event, occurrence, or interaction. It can be used to describe personal experiences, think about good past memories, or comment on the passage of time.