What is the time frame for Al Hamichya? If forgotten to say right after eating, how much time can pass before the bracha is not allowed to be said?
If one eats one’s fill of mezonos, for instance a pasta meal (which obligates al hamichyah, and not birkas hamazon), one should recite the after-blessing within 72 minutes of eating. If one delays past this time period, one may still recite the blessing provided one is still satiated from one’s eating, and one does not feel hungry for more.
If one eats a small amount of mezonos, one should recite the after-blessing immediately, because the period during which one may recite the after-blessing is not clear. Some write that one may certainly recite the blessing up to half an hour after eating, and some give the time as an hour. However, if one delays past this time, one may recite the berachah up to 72 minutes after eaing.
Sources: See Shulchan Aruch 164; Mishnah Berurah 184:20; Aruch Hashulchan 184:8; Shulchan Aruch Ha-Graz 184:3; Kaf Hachaim 184:28 (gives the time period of one hour); Veten Berachah (quoting from Rav Elyashiv that the preferred time is within half an hour); Vezos Haberachah pp. 49-50.