Optimally, one should drink all of the wine in the cup, which means at least a revi’it. If one uses a large cup that contains more than a revi’it, he should, le-khatĥila, drink all of the wine in the cup. If he does not want to, he should at least try to drink most of it. At the very least, one must drink most of a revi’it and a “melo lugmav.” In other words, there are two conditions: first, he must drink most of the wine that is required to be in the cup, i.e., most of a revi’it (38 ml or 76 ml in Ĥazon Ish shi’ur). Secondly, this amount must fill melo lugmav – enough wine to fill the drinker’s mouth with one cheek inflated. This is the amount of wine that settles one’s mind. For someone with a normal size mouth, melo lugmav a bit more than most of a revi’it. For one with a large mouth, melo lugmav is closer to a revi’it. A thirteen-year-old, whose mouth is small and whose melo lugmav is less than half a revi’it must drink most of a revi’it in order to fulfill the first condition (SA 472:9; MB ad loc. 30; BHL ad loc. s.v. “ve-yishteh”).
Children who have reached the age of mitzva education are given four cups of wine (SA 472:15). The age of education is when the child understands the meaning of the things that are said while the cups are filled – kiddush, the Hagada, Birkat Ha-mazon, and Hallel (SAH 472:25) – generally around age five or six. There is no need for them to drink most of a revi’it; melo lugmav suffices (MB 472:47).
One must drink most of the wine in the cup “at once.” It is obvious that if one drinks half of the necessary amount, and then, after a long pause, drinks the second half, he did not drink the necessary amount “at once” and thus did not fulfill drinking one of the four cups. However, leading Rishonim and Aĥaronim are divided over precisely how quickly one must drink the requisite amount of wine in order for it to be considered “at once.” According to Rambam, since people are accustomed to drink continuously, albeit with short breaks to breathe or swallow, one is only considered to have drunk “at once” if he drank continuously, as one would normally drink a revi’it. In other words, the amount of time one takes to drink most of a revi’it must be no longer than the amount of time it normally takes to drink an entire revi’it. If it takes one longer to drink most of a revi’it than it normally takes people to drink a revi’it, he has not fulfilled the mitzva according to Rambam. Raavad, on the other hand, maintains that as long as it does not take longer than the time it takes to eat a half a loaf of bread (“shi’ur akhilat pras”), i.e., within several minutes, it is still considered “at once.”
To summarize, one should preferably follow the stringent ruling of Rambam and drink most of the cup continuously, taking short breaks to breathe and swallow. Be-di’avad, if one drinks most of the wine in the cup within a shi’ur akhilat pras (6-7 minutes), he fulfills his obligation and need not drink the cup again, since the law of four cups is of rabbinic origin, and the halakha therefore follows the lenient position. Nonetheless, some practice stringency.
. According to Ramban, cited as an alternative opinion in SA 472:9, one must drink the majority of the cup, no matter how large. However, most poskim maintain that although it is preferable to follow Ramban, one fulfills the obligation without meeting that condition (MB ad loc. 33). Although the accepted opinion is the primary one, in this context I stressed that following the Ĥazon Ish shi’ur enhances the mitzva, since more wine adds to the mitzva of rejoicing and those who want to enhance the mitzva always drank more than the minimum revi’it.
Thus, to satisfy unquestionably all opinions one should preferably use a cup that contains 150 ml and drink most of it. This satisfies the higher melo lugmav requirement as well as Ramban’s condition. However, if it is difficult for him to drink an entire 150 ml “Ĥazon Ish shi’ur” cup, he ends up losing the enhancement of drinking an entire cup. Although we have stated that one may fulfill his obligation le-khatĥila by using the standard shi’ur, one who wishes to enhance the mitzva should first satisfy all opinions, including the Ĥazon Ish shi’ur, and only then undertake to drink an entire cup. If he enjoys drinking wine, the greatest enhancement is to drink a full 150 ml cup; if he is worried that he might become intoxicated, he may mix grape juice into the wine.
Optimally, one must drink uninterruptedly, as per Rambam, and if he drank the minimum amount within the time of akhilat pras, he has not fulfilled his obligation according to Rambam, only Raavad. But since we rule leniently when there is uncertainty pertaining to rabbinic law, one would not have to drink again. This is what AHS 472:13 and Ĥazon Ovadia §12 state. Moreover, according to Knesset Ha-gedola, SAH, and Ĥatam Sofer, Rambam only stated his opinion with regard to a forbidden drink, where one incurs lashes if he drinks continuously. But regarding berakhot over consumption of food or drink, Rambam would concur that shi’ur akhilat pras is the relevant time frame, since the determining factor of berakhot is enjoyment, and he certainly derives pleasure even if he only completes his drink during a shi’ur akhilat pras. Accordingly, perhaps Rambam would concur that one who drinks a cup of wine within a shi’ur akhilat pras would fulfill his obligation. Nevertheless, some poskim say that where it is easy to fulfill one’s obligation according to all opinions, one should be stringent even with regard to a disputed rabbinic law. Therefore, MA and SAH 472:20 and MB 472:34 state that if one drank the second cup only within the time of akhilat pras, he should drink it again. If it was the third or fourth cup, though, he should not drink again, since it would look like he was adding to the four cups. Rather, in this case he should rely on the opinion of Raavad that he has fulfilled his obligation (see MB 472:21 regarding the first cup). According to those who follow SA that one may drink after the third and fourth cups, one should re-drink any of the four cups in question, as Ben Ish Ĥai (Tzav 29) states. This is why I wrote that some are stringent in this matter, despite the fact that this is a double uncertainty about a rabbinic law: according to Raavad, the obligation has certainly been fulfilled, and possibly even according to Rambam as well.
See section 25 below and the notes ad loc. for the differing opinions regarding the length of shi’ur akhilat pras. The average time is around six to seven minutes, but le-khatĥila it is four minutes, and be-di’avad it is nine minutes.