On the night before Erev Pesah, one is required to perform Bedikat Hames – the search for Hames in his home (see Shulhan Aruch, Orah Haim 431). The time for the search begins at Set Ha'kochavim, or nightfall, on the eve of Erev Pesah. According to Hacham Ovadia Yosef, Set Ha'kochavim occurs in Israel approximately 20 minutes after sundown. In New York City, Set Ha'kochavim occurs at 35-40 minutes after sundown.
Halacha forbids performing certain activities from a half-hour before the time for the Bedika, until one completes the search. The Sages were concerned that a person who begins involving himself in these activities might forget to perform the Bedika, and they therefore forbade engaging in these activities already a half-hour prior to Set Ha'kochavim. Thus, for example, one may not perform Melacha – rigorous activity – during this period, such as fixing his car or doing work around the house. It is likewise forbidden during this period to eat bread or items requiring Mezonot (such as cake), in an amount exceeding a Ke'besa, or 2 oz. It is thus forbidden to eat a slice of pizza in this time, as a slice of pizza amounts to more than 2 oz. One may partake of lesser amounts of these foods, or of any amount of other foods, such as fruit, rice and the like.
The Shulhan Aruch also includes Torah study in his list of activities forbidden in this period. However, unlike the other activities, Torah learning is permissible within a half-hour prior to Set Ha'kochavim. Only with the advent of Set Ha'kochavim does it become forbidden to study Torah, given the possibility that one's involvement in learning will cause him to forget to search for Hames. This Halacha, however, applies only to private study. A group session, such as a Shiur delivered in the synagogue, may be held during this period, as the members of the group will undoubtedly remind each other to search for Hames after the learning session.
Summary: One must search his house for Hames on the night before Erev Pesah, anytime after Set Ha'kochavim (35-40 minutes after sundown, in the New York area). Starting a half-hour before this point, one may not perform intensive activity or eat more than 2 oz. of bread or other grain products, until he performs the search. From the time of Set Ha'kochavim, one may not study Torah in private until he performs the search.
