1. When the young boy’s mother refused to give him some candy, he started to cry. His mother told him he could cry until the cows come home, but he was not going to get any candy.
2. I will never be any good at giving speeches. I could practice until the cows come home , but I’ll never get over being nervous when I have to stand up in front of a crowd.

The expression relates to herds of cows that graze outdoors during the day and then are brought inside for the night. It means “all day long. ”