The Gemara establishes the famous rule of "En Kiddush Ela Bi’mkom Se’uda" – Kiddush must be recited in the framework of a meal. Meaning, if one did not eat after reciting Kiddush, then he has not fulfilled the obligation of Kiddush, because he did not recite Kiddush in the framework of a meal.
The Shulhan Aruch (Orah Haim 273) writes that one must eat "Tekef," or "immediately" after Kiddush. Clearly, this cannot mean the moment Kiddush is finished, because it takes some time to get up from the table, perform Netilat Yadayim, and so on. Moreover, the Gemara mentions a case of people who recited Kiddush in the basement and then ate the meal on the rooftop. Quite obviously, they could not have started their meal immediately after Kiddush. The Rama (Rav Moshe Isserles of Cracow, 1525-1572) therefore explains that it suffices if one had in mind when he recited Kiddush to begin the meal right away, or if one started the meal right away, even if he had not intended to do so. One does not fulfill the Misva only if he intended at the time he recited Kiddush to delay the meal, and he indeed delayed the meal.
The question, however, remains, how long of a delay disqualifies the Kiddush?
This issue is subject to a debate among the Halachic authorities. Hacham Bension Abba Shaul (Israel, 1923-1998) ruled that an intended delay of a half-hour disqualifies one’s Kiddush. He claims that in other contexts in Halacha the period of 30 minutes is considered "Samuch" – "near" – and thus if the Sages required eating one’s meal shortly after Kiddush, this must mean that the meal should begin within a half-hour of Kiddush. Hacham Ovadia Yosef, however, argues, and maintains that the meal must start within 72 minutes of Kiddush, as 72 minutes is the duration of "Ikul" – the process of digestion.
What precisely must one eat to be considered to have eaten a "meal" after Kiddush?
The Shulhan Aruch maintains that after Kiddush one must eat either a Ke’zayit of bread, or a Ke’zayit of "Mezonot" food made from one of the five principal grains. Although the Shilteh Gibborim (Rav Yehoshua Boaz, 16th century) ruled that one may also eat fruits, the Shulhan Aruch does not accept this view. Therefore, if a person attends a Kiddush in the synagogue, after hearing the recitation of Kiddush he must ensure to eat at least a Ke’zayit of bread or of "Mezonot" food (such as cake, cookies or crackers). If he only eats fruit or has a drink, he has not fulfilled his obligation. He will thus be considered as having eaten before Kiddush, which is forbidden, and will not allowed to eat when he gets home until he recites Kiddush.
The Shulhan Aruch also cites the opinion of the Geonim that it suffices to drink a Rebi’it – approximately 3.2 ounces – of wine. The Lebush (Rav Mordechai Yaffe, 1530-1612) understood this to mean that one must drink a Rebi’it in addition to the amount which he is required to drink for the Misva of Kiddush (a cheekful). This is also the view of the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Bereshit. Others, however, including the Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908) and Hacham Ovadia Yosef, disagree. They note that a Rebi’it suffices to require the recitation of a Beracha Aharona, and there is thus no reason to require drinking any more than this amount. Therefore, if one recited Kiddush, and instead of drinking only the minimum required amount of wine he drank a full Rebi’it, he has fulfilled the requirement of "Kiddush Bi’mkom Se’uda" even if he does not eat any bread or "Mezonot" food. It must be emphasized, however, that the people who heard Kiddush must still eat, even if the person who recited Kiddush drinks a Rebi’it of wine.
Rav Yosef Shalom Elyashiv (Israel, 1910-2012) ruled that although drinking a Rebi’it of wine suffices for this requirement, it does not suffice to drink a Rebi’it of grape juice. Since grape juice does not have the satiating effect that wine has, a Rebi’it of grape juice does not qualify as a "Se’uda" with respect to this Halacha. Hacham Ovadia Yosef, however, disagrees, and maintains that since grape juice indeed provides some degree of satiation, a Rebi’it of grape juice may be considered a "Se’uda" with respect to the Kiddush obligation.
Summary: In order to fulfill the obligation of Kiddush, one must eat a Ke’zayit of bread or "Mezonot" food, or drink a Rebi’it of wine or grape juice, immediately after Kiddush. If one recited Kiddush without the intention of eating within a period of 30 minutes (or, according to some opinions, 72 minutes) after Kiddush, and he indeed did not eat within that time, then he has not fulfilled his obligation.
