What does "when the cows come home" mean — and why is it funny?

informal, humorous

Meaning

For a very long time, or — more often — never; used to say something will take forever or simply will not happen.

Where it comes from

In use since at least the 1600s. The picture is of cattle ambling back to the barn at their own unhurried pace — that is, eventually, if ever.

Why it is funny

The humor is the lazy, hopeless slowness of the image. Cows are in no rush for anyone, so pinning your hopes to their schedule is a cheerful way of admitting the thing is never really happening.

Used in a sentence

"You can argue with him until the cows come home — he won't change his mind."