till the cows come home

till/until the cows come home (for a very long time) -- когда рак на горе свистнет; до морковкина заговенья; неопределнно долго, возможно, никогда...

Example:I could sit here and argue with you till the cows come home, but it wouldn't solve anything.

[когда рак на горе свистнет]
Just as it takes a while for the cows to come home from pasture on a farm, so it can seem like a long time when you are waiting for someone or something.