If a person ate a quantity of bread requiring Birkat Hamazon, but forgot to recite Birkat Hamazon immediately after eating, within how much time after eating may he still recite it? This question also arises in a situation of one who ate a food requiring the Beracha of Me’en Shalosh, such as if he ate a Kezayit of cake, and forgot to recite the Beracha. If he remembers later, may he still recite the Beracha Aharona?
A person in such a case may recite Birkat Hamazon or Me’en Shalosh when he remembers, provided that six hours have not passed since he finished eating, and that he still fills satiated as a result of the food which he ate. Both conditions must be met for him to recite Birkat Hamazon or the Me’en Shalosh. If six hours have passed since the time when he finished eating, or if he no longer feels satiated from the food, then he may no longer recite Birkat Hamazon or Me’en Shalosh. This Halacha emerges from the Mishna in Masechet Berachot (51) and the Gemara’s discussion of that Mishna (Daf 53).
In some situations, a person will not be able to determine whether he still feels satiated from the food. For example, if the person had eaten some other food in the interim, he has no way of knowing whether he feels satiated as a result of the original meal, or as a result of the food he ate afterward. In such a situation, one may recite Birkat Hamazon or the Beracha Aharona only if 72 minutes have not yet passed since the time he finished eating. After 72 minutes have passed, one may not recite Birkat Hamazon unless he is certain that he still feels satiated from the meal, and he is still within six hours of eating.
Summary: If a person forgot to recite Birkat Hamazon or Me’en Shalosh after eating, he may recite it when he remembers later, provided that he is still within six hours of eating, and he still feels satiated from the food. If he cannot determine whether he is still satiated from the food, such as if he ate some other food in the interim, then he may recite Birkat Hamazon or Me’en Shalosh only if he is within 72 minutes of eating.