Kitzur Shulchan Aruch, Rabbi Shlomo Ganzfried
Initially you are forbidden to leave your place or to engage in any activity until you have recited the after-berachah, lest you forget to say it. Nevertheless, if you left your place, and the berachah you [neglected] to say was Borei nefrshos rabbos, you may say it where you are now. But if you must say the "three-faceted berachah," then you must return to your place [where you have eaten] just as you would have to do it if it were Birkas Hamazon (see Ch. 44:9 above).
[If] after eating or drinking, you neglected to say the after-berachah immediately, you may recite the berachah until the food is digested, which is as long as you have no desire to eat fruit, and after drinking, as long as you are not thirsty again. After these time limits, you can no longer say the berachah. If you are unable to estimate [the time properly], you should, after remembering that you did not say the after-berachah, say a [pre-]berachah and eat some more of the same kind of food, and then say the after-berachah thereby exempting the food you ate before.
If someone ate or drank and vomited, the concluding berachah should not be said; as it is the same [as though] [the food] had already been digested.