Till the cows come home
For a long but indefinite time.
Cows are notoriously languid creatures and make their way home at their own unhurried pace. That's certainly the imagery behind 'till the cows come home' or 'until the cows come home', but the precise time and place of the coining of this colloquial phrase isn't known.
Francis Beaumont and John Fletcher published the comedy The Scornful Lady in 1616. The earliest printed version I have seen dates from 1717 and it includes this line.
Kiss till the Cow come home, kiss close, kiss close knaves.
Groucho Marx was never one to pass up an opportunity for a play on words and this occurs in his dialogue of the 1933 film Duck Soup:
"I could dance with you till the cows come home. Better still, I'll dance with the cows and you come home."