What exactly is Avraham saying? When did he first instruct Sarah to declare that they were brother and sister?
Rashi: When Hakadosh-Baruch-Hu took him away from his father's house to wander around from one unknown place to another - often one where Resha'im lived.
Rashbam: When Hashem exiled him from his land. 1
Targum Onkelos: When natons strayed after Avodah-Zarah, and Hashem brought him close to His service.
Targum Yonasan: When the people tried to lead him astray after their idol-worship, 2 but he left his father's house (to escape from their clutches).
Seforno: When the rampant (disgusting) idolatry forced him to leave his father's land to travel to a foreign land which he did not recognize.