Halakhic definition of location to follow a minhag

Question

Halakha says that a Jew must follow the minhag of the location where he lives. How far from the residence is considered the same location as the residence, in terms of minhag? Is it 2000 amot, 18 minutes, is the entire city considered a single location, or another parameter?

Answer 1

Bavli Chullin 110a

בסורא לא אכלי כחלי בפומבדיתא אכלי כחלי רמי בר תמרי דהוא רמי בר דיקולי מפומבדיתא איקלע לסורא במעלי יומא דכפורי אפקינהו כולי עלמא לכחלינהו שדינהו אזל איהו נקטינהו אכלינהו אייתוה לקמיה דרב חסדא אמר ליה אמאי תעביד הכי אמר ליה מאתרא דרב יהודה אנא דאכיל אמר ליה ולית לך נותנין עליו חומרי המקום שיצא משם וחומרי המקום שהלך לשם אמר ליה חוץ לתחום אכלתינהו ‏
The Gemara relates: In Sura they would not eat udders at all, even torn and roasted. But in Pumbedita they would eat udders. Rami bar Tamrei, who is also called Rami bar Dikulei, from Pumbedita, arrived in Sura on the eve of Yom Kippur. Since it is a mitzva to eat and drink then, large quantities of meat were cooked, and everyone brought out their udders from the animals they had slaughtered and threw them away. Rami bar Tamrei went and gathered the udders, roasted them, and ate them, in accordance with his custom. The residents of Sura brought Rami bar Tamrei before Rav Ḥisda, who said to him: Why did you do this? Rami bar Tamrei said to Rav Ḥisda: I am from the place of Rav Yehuda, who eats udders, and this is the accepted custom in Pumbedita. Rav Ḥisda said to him: And do you not hold by the principle that the Sages impose upon a traveler the stringencies of the place that he left and also the stringencies of the place to which he went? You should have accepted the stringency of Sura and not eaten the udders. Rami bar Tamrei said to Rav Ḥisda: That principle applies only to one who remains in the place he is visiting, but I ate the udders outside the boundaries of Sura.

The implication of the story is the rule you cite applies when one is in the same תחום (roughly 2000 cubits from the last house in the city).