פירוש על ברכות 8:7

שיתעכל המזון שבמעיו – As long as he is not hungry on account of that meal, it is a sign that the food was not consumed in his stomach. And specifically, [when] he forgot, the School of Hillel says that he should not go back, but if he did it on purpose, everyone agrees that he should return to the place [where he ate] and recite the blessing [of ברכת המזון].

Introduction This mishnah deals with a person who has forgotten to recite Birkat Hamazon after eating.

One who has eaten and forgotten to bless [Birkat Hamazon]: Bet Shammai says: he must return to the place where he ate and bless. But Bet Hillel says: he should say it in the place where he remembered. According to Bet Shammai one must return to the place one ate in order to recite Birkat Hamazon. Bet Hillel says that he can recite it wherever he might be.

Until when can he bless? Until sufficient time has passed for the food in his stomach to be digested. He may recite Birkat Hamazon as long as the food is still found in his stomach. According to the Talmud the food is considered to be in his stomach as long as he is not hungry. Once a person is hungry for a new meal we consider it as if the food is no longer in his stomach and he can no longer recite Birkat Hamazon.