When the cows come home

This is a very old English metaphor for a long, indefinite period of time, deriving from the long, slow time that it takes cows to return from the fields and meadows to their stables at the end of the day. The expression has been in use since the late 1500s, and its first citation is in a play called The Scornful Lady, published in 1616, written by Francis Beaumont and John Fletcher, in which someone is said to 'kiss till the cows come home.'