In order to recite an after-beracha —- take bread, for example —- must the full kazayit be eaten right after making the opening-beracha, or could one taste the bread and wait a period of time before eating the shiur kazayit (or two) in one sitting?
The STAR-K writes
Although ideally a kezayis should be consumed immediately, it is also acceptable for one to initially eat a small amount of bread after the brocha and then eat a kezayis of bread within a two, or at least four minute span later in the meal.
One is only obligated to make a Bracha Achrona on a food if one eats a Kezayit of food in Kedi Achilat Pras.
How long is Kedi Achilat Pras?
Shiurei Torah (Rav Chaim Noeh 3:15 pg 203) writes that Kedi Achilat Pras is 4 minutes.
Vezot HaBracha (pg 6) writes that many poskim (and quotes Rav Elyashiv as one of them) agree. See Aruch HaShulchan 202:8 and Kaf HaChaim 210:8.
Rav Ovadyah Yosef (Sh"t Yachave Daat 1:17) rules that Kedi Achilat Pras preferably is 4 minutes and after the fact (Bedieved) it is 7.5 minutes.
Or Letzion (vol 2, 14:17) holds that Kedi Achilat Pras preferably is 4 minutes and after the fact (Bedieved) it is 6 minutes.
So, ideally you have 4 minutes.