How long does a person have between the end of meal (eating has stopped) and when one can still bentsch? How much time must go by where benching is no longer effective for that meal? Please send sources. Thank you.
After having completed a meal, one should not unnecessarily delay saying Birkas Hamazon.However, it can be said up to 72 minutes after finishing the meal (eating).
If more than 72 minutes have elapsed (or if one is unsure as to how much time has elapsed), and he is now hungry again, Birkas Hamazon cannot be said. He should eat some more bread, and then bench.
Under ordinary circumstances, this will not require a new hamotzi blessing, but this depends on whether a full hefsek was made or not.
If more than 72 minutes have elapsed, but one is not hungry, he can still bench, but it is preferable to eat some more before benching.
See Shulchan Aruch (184:5) which gives the ruling that a person can bench until he becomes hungry again. The Mishnah Berurah (20; see Biur Halachah s.v. Im) writes that where one cannot tell if one is hungry again — for instance, where only a small amount was eaten — the time is 72 minutes. The Aruch Ha-Shulchan (7-8) writes that even for eating a large amount, it is difficult to gauge when one begins to be hungry again, and therefore one should always ensure that one recites Birkas Ha-Mazon within 72 minutes of finishing to eat.