Halacha Orach Chaim 184

We learned that one must recite Birchas Hamazon before becoming hungry once again. What do we mean when we say hungry?

The Mechaber says that hungry means the food began digesting to the point that one is no longer full. There is a disagreement among the Achronim if non bread foods are viewed as a continuation of the meal, and will therefore extend the time Birchas Hamazon can be recited. If one is sitting by the table, and has not formally ended the meal, there is also reason to permit Benching even if there is a question if the time has already transpired.

If someone is not sure if he is hungry, the Mishna Berura rules that one can still Bench within the amount of time it takes to travel 4 Mil. The accepted time frame for this is 72. This window applies to someone that is hungry right after eating, but does not have any more food to eat as well. Technically, he should be required to recite Birchas Hamazon right away. However, the Chofetz Chaim writes, that if he inappropriately delayed, Birchas Hamazon can still be recited within the amount of time it takes to travel 4 Mil. The standard time it takes to travel 4 Mil is 72 minutes, but, under extenuating circumstances, th Biyur Halacha would allocate 24 minutes per Mil, or 96 minutes.