One who plans on eating a Ke’besa – approximately 2 oz. – or more of bread must wash Netilat Yadayim with a Beracha. Within how much time after washing his hands must he eat this quantity of bread in order to justify the Beracha that he recited over the Netilat Yadayim?
Normally, when Halacha requires the consumption of a Ke’zayit (half a Ke’besa), it requires that one eat the Ke’zayit within a time-frame known as "Kedeh Achilat Pares," which, according to Hacham Ovadia Yosef, is 7.5 minutes. In the case under discussion, then, when one must eat twice this amount, each Ke’zayit must be eaten within 7.5 minutes. Moreover, there must be less than a 7.5-minute interruption between the consumption of the two Ke’zetim. And thus, for example, if a person eats the first Ke’zayit in six minutes, and then three minutes later he begins eating the second Ke’zayit, which he eats in about four minutes, then he has justified the Beracha he had recited. But if a person eats the first Ke’zayit, and then waits until the end of the meal before eating the second Ke’zayit, he has not eaten a Ke’besa within the required time-frame, and the Beracha he recited over Netilat Yadayim thus becomes illegitimate. It is therefore advisable after washing Netilat Yadayim to eat the full quantity of 2 oz. of bread immediately, without any delay, to ensure that the Beracha he had recited is valid.
This Halacha appears in Yalkut Yosef – Berachot, vol. 2, p. 77 (in the footnote).
Summary: One recites the Beracha over Netilat Yadayim only if he will be eating 2 oz. of bread, and each ounce will be eaten within 7.5 minutes, without a 7.5-minute interruption between the two ounces. It is therefore advisable to eat 2 oz. of bread immediately when one begins the meal to ensure that the Beracha does not retroactively become a wasted Beracha.
