Use the phrase until the cows come home to mean "for a long, long time." You can argue with your sister until the cows come home, but you'll never convince her vanilla ice cream is better than chocolate.
The expression until the cows come home has been around for centuries, first appearing in print in the 1500s (although it originally referred to a single cow coming home). If you've ever watched a herd of cows meander across a field, so slowly it seems they'll never reach the barn, you'll get a sense of the phrase's origin. If your math homework is taking forever to finish, you'll be working on it until the cows come home.