אורח חיים תל״אOrach Chayim 431

Se’if 1א׳

בִּתְחִלַּת לֵיל י"ד בְּנִיסָן בּוֹדְקִים אֶת הֶחָמֵץ לְאוֹר הַנֵּר, בַּחֹרִין וּבַסְדָקִין, בְּכָל הַמְּקוֹמוֹת שֶׁדֶּרֶךְ לְהַכְנִיס שָׁם חָמֵץ.

The Time for the Search of Chametz, 2 Seifim: At the beginning of the night of the 14th of Nisan, we search for chametz by the light of a candle in holes and cracks in every place that it would be usual to have brought chametz.

Se’if 2ב׳

יִזָּהֵר כָּל אָדָם שֶׁלֹּא יַתְחִיל בְּשׁוּם מְלָאכָה וְלֹא יֹאכַל, עַד שֶׁיִּבְדֹּק. וַאֲפִלּוּ אִם יֵשׁ לוֹ עֵת קָבוּעַ לִלְמֹד, לֹא יִלְמֹד עַד שֶׁיִּבְדֹּק; וְאִם הִתְחִיל לִלְמֹד מִבְּעוֹד יוֹם, אֵין צָרִיךְ לְהַפְסִיק. וְיֵשׁ אוֹמְרִים שֶׁצָּרִיךְ לְהַפְסִיק, וְכֵן נִרְאֶה לִי עִקָּר.

Every person must be careful not to start any work or to eat until they have searched. Even if they have a fixed time to study, they should not study until they have searched. If they started learning while it was still day, they don't have to stop ( and some say they do have to stop, which seems correct to me)