The obligation of Mezuza requires a person to affix Mezuzot on the residence in which he lives. The Gemara describes this obligation as "Hobat Ha’dar" (literally, "an obligation of the resident"), which means that the Misva applies only to the resident. If a person purchases a property but it remains vacant for a period of time, it does not require Mezuzot. It is only when a person moves into a residence that he must affix Mezuzot.
This Halacha led Rabbi Akiva Eiger (1761-1837) to an interesting discussion in one of his published responsa (Siman 9; listen to audio recording for precise citation). If a person leaves his home for an extended period of time, such as for summer vacation or an extended business trip, and his family travels with him, such that the house is empty, the obligation of Mezuza seemingly does not apply to the home during that period. Since nobody resides in the house, there is no obligation of Mezuza. The Misva resurfaces when the family returns and once again takes residence in the house. It would thus appear, Rabbi Akiva Eiger notes, that one must recite a Beracha, "Al Misvat Mezuza," when he returns from an extended absence. This situation should be no different than the case of a person who leaves his Sukka for the day on Sukkot, who must recite a Beracha when he returns to the Sukka to eat dinner. The Misva ended, so-to-speak, when he left the Sukka, and he then fulfilled it again upon his return, thus warranting a new Beracha. Likewise, when a person leaves his home for an extended period, the Misva of Mezuza does not obtain, and he then fulfills it again when he returns home, and must therefore recite a Beracha.
However, as Rabbi Akiva Eiger notes, the Hid"a (Rav Haim Yosef David Azulai, 1724-1806), in his Birkeh Yosef (Orah Haim 9), writes that the Sages instituted the Beracha of Mezuza to be recited only when one affixes the Mezuza. Thus, according to this reasoning, one would not recite a Beracha on the Mezuza when he returns from an extended absence.
Rabbi Akiva Eiger leaves this as an open question, but other authorities ruled definitively that one should not recite a Beracha when he returns home. Rabbenu Yehonatan, one of the Rishonim (Medieval Halachic scholars), noted that the obligation of Mezuza applies even to residences where a person lives on an occasional basis. Even though he does not live in the residence most of the time, it nevertheless requires Mezuzot since he uses it from time to time. Certainly, then, the obligation of Mezuza continues to apply to one’s primary residence even during trips and vacations when he is not actually in the home. Hence, one does not recite a new Beracha when he returns home after an extended absence. This is the ruling of Hacham Yishak Yosef, in his work Yalkut Yosef.
Summary: One recites the Beracha over the Mezuza only when he affixes it to the doorframe. The Beracha is not recited when a person returns home after being away for an extended period.
