The expression ‘until the cows come home’, means ‘for a very long time’.
The cowherd takes the cows out for grazing early in the morning and brings them back only in the evening. He is out the whole day and hence the term is used to mean ‘for a very long time’.
When the expression was first used, it meant ‘entire day’. Now it’s used to mean for a very long time.
Example: “ You can keep asking till the cows come home, but there is no way that you are going to get a new laptop.
“You are waiting for my boss to come up with a reasonably good idea? Then I’m afraid you’ll be waiting until the cows come home.”
“The two parties debated the issue till the cows came home, but for some strange reason they just couldn’t reach an agreement.”
Source: “The Hindu” – Know Your English Column – March 04, 2008