Also, when the cows come home. For a long time, as in You can keep asking till the cows come home, but you still may not go bungee-jumping. This term alludes to when the cows return to the barn for milking. [Late 1500s]

For a long time: “Mr. Rowland said that as far as he was concerned, the delinquent students could stay there washing blackboards till the cows came home.”