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. It was certainly before 1829 though, and may well have been in Scotland. The phrase appeared in print in The Times in January that year, when the paper reported a suggestion of what the Duke of Wellington should do if he wanted to maintain a place as a minister in Peel's cabinet:


If the Duke will but do what he unquestionably can do, and propose a Catholic Bill with securities, he may be Minister, as they say in Scotland "until the cows come home."