The Shulhan Aruch (Orah Haim 210) rules that a Beracha Aharona is required after one eats a "Ke’zayit: of food within the time-period of "Kedeh Achilat Paress" – which means the average amount of time needed to eat three "Ke’beitzim" of bread. If a person eats a "Ke’zayit" or more, but over the course of a longer period than "Kedeh Achilat Paress," he does not recite a Beracha Aharona, or, in the case of eating bread, he does not recite Birkat Ha’mazon.
There is considerable discussion among the Poskim as to the precise length of this period, "Kedeh Achilat Paress." The Kaf Ha’haim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1870-1939) experimented by eating this volume of bread on several occasions to determine the duration of "Kedeh Achilat Paress," and he found that eating this amount of bread takes approximately 4-4.5 minutes. Accordingly, Hacham Bension Abba Shaul (Israel, 1924-1998) ruled that a "Ke’zayit" needs to be eaten within four minutes for a Beracha Aharona to be required. Hacham Ovadia Yosef, however, based on the writings of Rav Haim Palachi (Turkey, 1788-1868), reached a different conclusion, ruling that the "Ke’zayit" must be eaten within a period of 7.5 minutes for a Beracha Aharona to be required.
The definition of "Kedeh Achilat Paress" has other ramifications, as well. For example, at the Shabbat meal, one is required to eat a "Ke’zayit" of bread within this time-frame. Some people eat a very small piece of bread at the beginning of the meal, when the Beracha is recited over the bread, and then eat some more later on. This is incorrect; one must ensure at the Shabbat meal to eat a "Ke’zayit" within a period of 7.5 minutes.
Hacham Ovadia added that although in general one can follow the definition of 7.5 minutes, when it comes to Torah obligations to eat, one should endeavor to eat the "Ke’zayit" within four minutes. On the first night of Sukkot, therefore, when we are required by force of Torah obligation to eat a "Ke’zayit" of bread in the Sukka, one should try to eat this amount within a period of four minutes. This applies also on the first night of Pesach, when the Torah requires eating a "Ke’zayit" of Masa. One should try to eat the "Ke’zayit" within four minutes.
There are instances where Halacha requires eating a "Ke’besa," which is twice the amount of a "Ke’zayit" (generally assumed to be 2 oz.). For example, after washing Netilat Yadayim with a Beracha, one should eat a "Ke’besa" of bread. When a "Ke’besa" must be eaten, we simply double the duration of "Kedeh Achilat Paress." Therefore, after washing Netilat Yadayim with a Beracha, one should eat 2 oz. of bread within a period of 15 minutes. On the first night of Sukkot, when it is preferable to eat at least 2 oz. of bread within the shorter definition of "Kedeh Achilat Paress," one should try to eat this amount within eight minutes.
Another application of this rule is the Halacha requiring one to recite Birkat Ha’mazon if he ate four "Ke’besot" – or eight "Ke’zetim" – of "Mezonot" food. This Halacha applies if a person ate this quantity of "Mezonot" food (such as cake or cookies) within a time-frame of about an hour (8 * 7.5). If it takes him longer to eat 8 oz. of "Mezonot" food, then he does not recite Birkat Ha’mazon, and instead recites a Beracha Aharona (assuming he had eaten a "Ke’zayit" within 7.5 minutes).
Summary: One who ate a "Ke’zayit" of food within a period of 7.5 minutes recites a Beracha Aharona (or, in the case of bread, Birkat Ha’mazon); if it took longer that this amount of time, one does not recite a Beracha Aharona. After one washes Netilat Yadayim for bread with a Beracha, he should ensure to eat a "Ke'besa" of bread within 15 minutes. On the first night of Sukkot, one should eat a "Ke’zayit" of bread within four minutes, and on the first night of Pesach, one should eat a "Ke’zayit" of Masa within four minutes. One who eats 8 oz. of "Mezonot" food within an hour recites Birkat Ha’mazon, instead of "Al Ha’mihya."
