Origin of the phrase "tell me when"

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?

Not only is it common, it even has a dictionary mention, however this mentions Say when rather than Tell me when and that's certainly what I have encountered most often (and use myself).

when, adv. (conj. and n.)

2. In an indirect question or clause of similar meaning: At what time; on what occasion; in what case or circumstances. Also ellipt.
say when, colloq. formula used by a person pouring out drink for another, to ask him to say when he shall stop; also ellipt., as a reply to this formula.

[OED]

That is, in answer to the colloquial formula "Say when", a person might answer "When!" as a humorous answer to being told to say that, as well as indicating "Now is when I want you to stop".