Where Did the Phrase "Until the Cows Come Home" Originate?

Question

as in "Grandma can knit and knit 'til the cows come home!"

Answer 1

In some parts of the world animals are taken into the mountains to graze during the summer when the grass there is growing well.  They stay there until the weather starts to get cold and are brought down to pastures at lower altitudes (many of which have been used in the animals' absence to grow hay for storage and feeding during the winter months). Thus, the period of time that the animals are gone from their home farms is a long time, so "until the cows come home" means a long time.

I would imagine some variant of the expression first originated in countries in and adjacent to the Alps and made its way into the British Isles, where I doubt cattle were ever sent up into any hills or mountains in summer.