Apparently:
Answer: It's an old docker's term. When people put things on a ship, they used a crank called a donkey. They were very slow and when they were asked how long it would take, they said “donkey's years”.
I've no idea if it's true but it certainly sounds plausible.
And in commercial kitchens the chefs often use a dolly, which is a trolley for putting food trays on. I enjoy this kind of thing. Hope I'm not boring you. (But no one is forcing you to read my postings.
)