WNW: Until the Cows Come Home

One curious (dare I say word-nerdy?) reader asked where the phrase until the cows come home comes from.
I had no idea, but that's exactly the kind of random thing I like to find out.
We all know the basic gist of the phrase: partying or doing something else for a really long time, such as late into the night.
But where do the cows fit in? And why use cows when they're such lazy, unmotivated creatures?
Ah, so that's where my handy research came in.
Cows are lazy, yes. But there is one time...