Growing up in my family, we would often use the phrase "tell me when" when serving each other food, pouring drinks, etc. For example, my mother would begin pouring me a glass of milk and say "tell me when"--I would then say "when" to indicate that she should stop pouring.
When I got married, my wife was familiar with the phrase, and I had always assumed this was a common phrase. However, I don't remember using it much (if at all) outside of home. Is the usage of this phrase common? What are its origins?