Chapter 44: 7-9
Reciting Grace after Meals7. It is customary that the people listening to grace respond "Amen" after the phrases beginning Horachomon... (The Merciful One...). This is based on the Midrash which teaches that whenever one hears a person recite a prayer or bless Israel, he is obligated to respond "Amen," even though G-d's name was not mentioned.
8. A person who transgresses and delays his recitation of grace until after he has begun to digest his meal can no longer recite grace. Our Sages described this measure as the time it takes for a person to become hungry again. Some authorities have fixed this measure at 72 minutes.* However, people will often sit together for longer than that measure after large meals before they recite grace. This is acceptable, for in the interim they are still drinking and eating dessert. Nevertheless, it is proper not to delay the recitation of grace for a prolonged period of time.
* {The Shulchon Oruch HoRav (184:3) and the Mishnoh B'rurah (184:20) allow a person who ate a full meal to recite grace even if he delayed a longer time. Nevertheless, this applies only after the fact. By no means should a person intentionally delay the recitation of grace.}
9. When a person transgresses and leaves the place where he ate before reciting the grace, if there is some bread at the place where he is, he may eat there - without reciting the blessing hamotzi (as stated in Chapter 42, Law 19) - and afterwards recite grace there.
If he has no bread there, he must return to the place where he ate to recite the grace. If he is so far from the place he ate that it is possible that, in the time it takes him to return, his food will begin to be digested [and thus he will no longer be allowed to recite grace, as stated above], he may recite the grace in his present location.