K"day Achilas Pras - Some Achronim pasken that the shiur of k'day achilas pras is 6 minutes. The Aruch Hashulchan writes that the shiur is equal to 3 or 4 minutes. The Marcheshes writes that the accepted time is 3.6 minutes, however, R'Moshe Feinstein (O"C 4:41) paskens that one must make sure to eat a k'zayis of bread within 3 minutes in order to recite birchas hamazon.
K'day Shtiyas Riviis - Many Achronim hold that this timeframe is the same as K'day Achilas Pras. Other Achronim including the Mishna Brura (Shaar Hatziyon 210:11) hold that one must drink a reviis in a maximum of two gulps with a short pause between the two gulps.
Shiur Ikul - R' Bodner in his sefer on brachos writes that for small quantities of food one has 30 minutes until he can no longer recite a bracha achrona. For larger bread meals, one generally has 72 minutes until he can no longer recite birchas hamazon for the k'zayis. The Kaf HaChaim (184:28) paskens, that if 72 minutes have passed and you still do not feel hungry you may recite birchas hamazon. If you are unsure whether or not you are still hungry, R' Bodner writes that you should eat another k'zayis of bread without a new hamotzei and then recite birchas hamazon. If one can not eat another k'zayis and is unsure if he is hungry, he may not recite birchas hamazon.