Till the cows come home

You milk cows in the morning, quite early. For this you bring them indoors. You milk them again in the evening, quite late. For this you bring them indoors. In between those times, the cows are often outdoors, eating grass. So as to make the milk, you see.

Therefore, til the cows come home means until you bring them in for the evening milking. All day, basically. And hence, a long time.