As we know, Halacha requires reciting a Beracha after eating a certain quantity of food, or drinking a certain quantity of liquid. For example, one who eats a "Ke’zayit" or more of bread recites Birkat Ha’mazon, one who eats a "Ke’zayit" or more of "Mezonot" food recites "Al Ha’mihya," and one who drinks a "Rebi’it" or more of water recites "Boreh Nefashot."
What is less known, however, is that the Beracha must be recited within a certain time period. Specifically, it must be recited before the food has been digested. Once a person no longer feels satiated from the food he consumed, he can no longer recite the required Beracha.
When it comes to bread, it is generally assumed that one can recite Birkat Ha’mazon within 72 minutes of eating. However, people with slow digestion, who still feel satiated even hours later, can recite Birkat Ha’mazon even well beyond this point.
When it comes to water and other beverages, however, there is considerable discussion among the Poskim as to the duration of time one has after drinking to recite "Boreh Nefashot." Some Poskim maintained that the Beracha may be recited within a half-hour of drinking, but others ruled that the Beracha must be recited within two minutes, because often, especially in the summertime, when one perspires, the water one drinks is digested very rapidly in the body. It is not uncommon in the summer – particularly if one is playing sports or exercising – for one to feel thirsty within minutes of taking a drink, and his renewed thirst indicates that the water he had drunk has already been fully processed, such that he can no longer recite "Boreh Nefashot."
For this reason, Hacham Bension Abba Shaul (Israel, 1924-1998) ruled that one should try to recite "Boreh Nefashot" immediately after drinking, to ensure he recites the Beracha within the required time frame. Even if one will need to drink again soon afterward, he should still recite "Boreh Nefashot" now, after drinking, because the water he drank might go through his body’s system very rapidly, and he will then have missed the time frame for the recitation of the Beracha.
This applies also to the Beracha of "Al Ha’gefen" which one recites after drinking a "Rebi’it" of wine. If one drinks wine at a Kiddush on Shabbat morning, for example, he should not wait too long to recite "Al Ha’gefen," because it does not take all that long for the wine to go through his system. Even if he still feels full from the food he ate at the Kiddush, nevertheless, the Beracha of "Al Ha’gefen" is required for the wine which he drank, not for the food, and so it must be recited before the wine has been fully digested. Therefore, like "Boreh Nefashot" after drinking other beverages, "Al Ha’gefen" must be recited as soon as possible after one has drunk wine.
Summary: It is not entirely clear how much time one has to recite a Beracha Aharona after drinking ("Al Ha’gefen" after wine, "Boreh Nefashot" after other beverages), and therefore, one should recite the Beracha Aharona immediately after drinking.
