Timely invitation to put a limit on drink (3,4)

I believe the answer is:

say when

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I'm afraid I can't explain this answer.

(Other definitions for say when that I've seen before include "Request to be told to stop pouring drink" , "Tell me when to stop pouring" , "Words from the pourer?" , "Pourer's words" .)

I've seen this clue in the Evening Standard.

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