51:13 After eating something, we’re not supposed to leave our place or get involved with some other activity until we’ve recited the concluding bracha, as a safeguard against forgetting. (One may leave with the intention of returning immediately – Shaarei Tziyon 178:26.) However, if one did leave his place, he can recite borei nefashos in his new location. Since the rules for the bracha mei’ein shalosh are like those for bentching, he must return to his original location in order to recite al hamichya, al hagafen or al ho’eitz.
51:14 If someone did not immediately recite a concluding bracha, he may do so for the time it takes to digest, namely as long as he is not yet hungry for fruit or, in the case of drinks, thirsty. After this, he may no longer recite concluding brachos. If someone who cannot estimate these times realizes that he did not recite the concluding bracha, he should say a bracha on the same type of food that he already ate and say a concluding bracha that includes both this item and the previous one.