Halacha requires drinking at least a "Melo Lugmav" (cheekful) – or about 1.7 ounces – of the wine in the Kiddush cup. Preferably, the one who recites Kiddush should drink this amount, but if he is cannot do so, for whatever reason, then he may give the wine to somebody else, who heard the Kiddush, to drink.
There is a debate among the Halachic authorities concerning a case where nobody drank this required amount, for example, if the one who recited Kiddush took a small sip and then put the cup down, but nobody else drank from it. Several Halachic authorities, including the Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908) and Hacham Bension Abba Shaul (Israel, 1923-1998), understood the Bet Yosef (Orah Haim 271) as indicating that the obligation of Kiddush is fulfilled even if the wine is not drunk. Although the "Misva Min Ha’mubhar" – the highest standard of performing the Misva – requires drinking a "Melo Lugmav" of wine, the basic obligation is fulfilled even if nobody drinks. Therefore, if it happened that nobody drank the required amount of Kiddush wine, the obligation was fulfilled and one does not have to repeat Kiddush. Hacham Bension adds, however, that a person in this case should try to hear Kiddush from somebody else, but strictly speaking, this is not required, as he has fulfilled his obligation. Hacham Bension applies this ruling to Habdala, as well; if nobody drank a Rebi’it of wine, the obligation has nevertheless been fulfilled.
Hacham Ovadia Yosef, in Hazon Ovadia – Shabbat, disagreed, and understood the Bet Yosef’s comments differently. According to his reading, one does not fulfill even the basic obligation of Kiddush if nobody drinks the minimum required amount of wine. This is also the position of the Mishna Berura.
Rabbi Moshe Halevi (Israel, 1961-2001), in Menuhat Ahaba, writes that it suffices even if somebody who heard Kiddush drank a "Melo Lugmav" of wine from his own cup. Meaning, even if nobody drank the wine from the cup upon which Kiddush was recited, nevertheless, if at least one person at the table drinks a "Melo Lugmav" from his cup, then everyone has fulfilled their obligation. Therefore, if one is listening to Kiddush and sees that the person who recites Kiddush is not planning to drink the wine, he should drink his own cup of wine, and in this way everyone has fulfilled the obligation.
Summary: After the recitation of Kiddush, somebody – preferably the one who recited Kiddush – must drink at least 1.7 ounces from the Kiddush cup. If nobody drinks from the Kiddush cup, it suffices if somebody at the table drank this amount of wine from his own cup. According to some opinions, the Misva is fulfilled even if nobody drank any wine, though even according to this view, a person in this situation should try to hear Kiddush again.
