Within how much time must the Kezayis of Matzah be eaten?

Lechatchila:[1] Initially, one is to eat the entire Kezayis in one shot. One is thus to place the entire Kezayis in his mouth simultaneously, chew it until it becomes crumbs, and then swallow the entire Kezayis together. [Practically, this was only possible in previous times when the Matzah was made of soft bread like texture, however todays Matzah of crunchy cracker like texture does not give ability to enter a full Kezayis into ones mouth simultaneously, and hence one is to follow the Bedieved opinion brought next.[2]]   

Bedieved:[3] Bedieved, one fulfills his obligation of eating Matzah even if he did not eat the entire Kezayis consecutively, such as if he ate half of the Kezayis, delayed, and then ate the other half of the Kezayis. However, the entire Kezayis of Matzah must be eaten within the amount of time Halachically defined as “Kdei Achilas Peras” in order for one to fulfill his obligation. [This is approximately within 3-4 minutes.[4] However, a child, and the sick and elderly may be lenient up until nine minutes.[5] See Chapter 3 Halacha 6 for the full details of this subject!] If one did not eat a Kezayis of Matzah within this amount of time, then one does not fulfill his obligation.

[1] Admur 475:6 and 14 “Even though Lechatchila one must eat the entire Kezayis simultaneously”; M”A 475:4; Taz 475:11; Chok Yaakov 475:22; Terumas Hadeshen 139; Kaf Hachaim 475:67

Other opinions: Some Poskim rule that even initially one may eat the Kezayis of Matzah a little bit at a time, and there is no need to consume the entire Kezayis simultaneously. [Opinion brought in Admur 475:14 regarding maror; M”A 475:4 in name of Maharil Seder Hagdah p. 110 even regaridng Matzah]

[2] See other opinions brought in previous footnote and Admur 475:14 that if it is difficult, one may even initially rely on this opinion

[3] Admur 475:8 “Bedieved…even if the Kezayis of Matzah was eaten a little at a time, in many sessions, so long as the time of Achilas Peras, which is three Beitzim, did not pass from the start of the first eating and the end of the last eating, then he is Yotzei.”; Michaber 475:6; Pesachim 114b; M”A 475:4 and 15 in name of Maharil Seder Haggadah; Chok Yaakov 475:22; Elya Raba 475:11; Peri Chadash 475; Birkeiy Yosef 475:5; M”B 475:42; Kaf Hachaim 475:68; See Brachos 37b

Other opinions: Some Poskim rule that so long as one does not wait Kdei Achilas Peras between the two sessions of eating, one is Yotzei, even if the eating of the Kezayis takes more than Kdei Achilas Peras. [Chok Yosef, brought and negated in Birkeiy Yosef 475:5]

[4] Conclusion of Rebbe in Sefer Haminhagim [English] p. 93 to suspect for three minutes; Conclusion of four minutes: Shiureiy Torah 3:15 [p. 303] and Piskeiy Teshuvos 210:1 that so is the widespread custom; See Piskeiy Teshuvos 475 footnote 53 that if one is in doubt if he ate it within 4 minutes then he is not required to re-eat, and may rely on the other opinions; For a full overview on this subject, See: Shiurei Torah 3:15 for the following other opinions mentioned: 4.5, 5, 6, 7.5, 8, 9; See Sefer Haminhagim ibid and footnotes 342-344; Piskeiy Teshuvos 158:7 and 210:1; 271:9; 475:10; The following are the various opinions in Poskim:

Two minutes: Some Poskim rule that Kdei Achilas Peras is 2 minutes. [Rav Y.Y. Kanievsky in Shiuran Shel Torah Shiureiy Hamitzvos 30]

Three minutes: Some Poskim rule that Kdei Achilas Peras is 3 minutes. [Igros Moshe 4:41 in name of Marcheshes; Orchos Rabbeinu p. 70 that so was custom of Chazon Ish; Sefer Haminhagim ibid and footnotes 342-344 in tradition from the Tzemach Tzedek, in name of Rav Hillel Miparitch, that the minimum of Achilas Peras is 3 minutes and maximum 7 minutes. The Rebbe concludes there in the footnote that one is required to suspect for the above tradition.]

Four minutes: Some Poskim rule that Kdei Achilas Peras is 4 minutes. [Shiurei Torah 3:15 [p. 303]; Aruch Hashulchan 202:8; Kaf Hachaim 210:5; Piskeiy Teshuvos 210:1 that so is the widespread custom]

Opinion of 6-8 minutes: Some Poskim rule that the Shiur of Kdei Achilas Peras is 6-7 minutes. [Tzemach Tzedek Shaar Hamiluim 1:8-10, brought in Ketzos Hashulchan 36 footnote 5; 59 footnote 4; Shiureiy Torah ibid footnote 35; Sefer Haminhaghim [English] p. 93 regarding Tishe Beav] Other Poskim rule that the Shiur of Kdei Achilas Peras for 4 Kebeitzim [i.e. Yom Kippur] is 8 minutes, while the Shiur for eating Mitzvos or after blessing is 6 minutes. [Gulot Ariel Chidushei Mishnayos Mikvaos 10:7 in name of Alter Rebbe “I received in the name of my great uncle, the Alter Rebbe, that the Shiur is eight minutes, as he measured the eating an egg by an average person and it took two minutes, and so is my custom to rule”]

Opinion of 9 minutes: Some Poskim rule that the Shiur of Kdei Achilas Peras is 9 minutes. [See Shiurei Torah 3:15; Piskeiy Teshuvos ibid]

Chabad custom: See Gulot Ariel Chidushei Mishnayos Mikvaos 10:7 in name of Alter Rebbe that the Shiur is two minutes per Beitza for a total of 6 minutes for food Mitzvos and 8 minutes for Yom Kippur [See Shiuriei Torah p. 204]; See Sefer Haminhagim ibid and footnotes 342-344 for a quote of various opinions of Achilas Peras, and for a tradition from the Tzemach Tzedek, in name of Rav Hillel Miparitch, that differs from the response of the Tzemach Tzedek which placed Achilas Peras as 6-7 minutes and rather places Achilas Peras as minimum 3 minutes and maximum 7 minutes. The Rebbe concludes there in the footnote that one is required to suspect for the above tradition. Accordingly, one should try to eat the Kezayis within three minutes. However, if one ate it within four minutes, and perhaps even within 6-7 minutes, one can rely on the Tzemach Tzedek’s other rulings. Vetzaruch Iyun, as perhaps Safek Brachos Lihakel. In any event one is to be very careful in the above.]

[5] Piskeiy Teshuvos ibid in name of “all Poskim”