say when
verb1
Idiomatic An imperative form used to request that the interlocutor indicate when one should stop doing something, esp. pouring a drink, because one has reached a sufficient amount.
- Say when! ― When! Thanks.
- The publican placed a whisky bottle and three glasses on the ledge and Grierson poured, with the courtesy of, "Say when," to the trooper, who was forced to exclaim, "Hold on!" in protest at a tumbler three-quarters full of whisky.1938, Norman Lindsay, Age of Consent, 1st Australian edition, Sydney, N.S.W.: Ure Smith, published 1962, →OCLC, page 161: