until the cows come home

Apparently from the fact that cows may be expected to return for milking in the morning; thus, for example, a party that goes on "until the cows come home" is a very long one. Or as the usage is Scottish, cows in the highlands are put out on the common grazing until the end of summer when they will find their way back home to be let in for feeding. There is plenty of grass on the hill (common), so cows can stay out for months until the scarcity of food brings them home in the fall.