Wiktionary
alt. (context idiomatic English) For a very long period of time. prep.phr. (context idiomatic English) For a very long period of time.
Usage examples of "until the cows come home".
The machine gun probably can't depress far enough to shoot down at him, but they can roll grenades at him until the cows come home, or just pick him off with small arms as he gropes for handholds.
So you get another job, and you finally find a place where they treat you right, and one day you notice you haven't been on an audition or a go-see in months, and for a while you feel guilty about it, and then you feel guilty that you don't, and then that's it, you're a lifer, you'll be mixing Salty Dogs and Harvey Wallbangers until the cows come home.
She will slice and dice exactly as she pleases, pick apart to her heart's content and fuss with putting back together until the cows come home.
Dannyboy is simply mad for the subject of immortality and will yak about it until the cows come home, although the precise time and date of bovine arrival has yet to be reckoned to his satisfaction.