There is a Torah obligation to eat a Ke'zayit of bread in the Sukka on the first night of Sukkot. Ideally one should eat a Ke'besa (twice the volume of a Ke'zayit) to fulfill this obligation, but strictly speaking it suffices to eat a Ke'zayit. This Ke'zayit should be eaten in a single sitting, without any interruption.
One must eat the Ke'zayit of bread after Set Ha'kochavim (nightfall), when Yom Tov has begun according to all opinions. Even if one has already recited Arvit, he should eat the Ke'zayit of bread only after Set Ha'kochavim. Set Ha'kochavim occurs (in the New York City area) approximately 40 minutes after sundown. Those who wish to act stringently in accordance with the view of Rabbenu Tam (France, 1100-1171) should wait 72 minutes after sundown. It is indeed advisable to act stringently in this regard as we deal with a Torah obligation. If delaying the meal will cause one's family difficulty or discomfort, then he can begin the meal earlier and then make a point to eat a Ke'zayit of bread sometime after 72 minutes have passed since sundown.
The Ke'zayit of bread should be eaten before Hasot (midnight as defined by Halacha). If one was unable to partake of the Ke'zayit of bread before Hasot, or if rain fell until after Hasot and one therefore ate indoors, he should eat another Ke'zayit in the Sukka after Hasot. In such a case, however, he does not recite the Beracha of "Lei'shev Ba'Sukka" before eating unless he eats a Ke'besa.
(Taken from Hacham Ovadia Yosef's rulings in Hazon Ovadia Laws of Sukkot, page 108, Halacha 4)
