If one moves into a new apartment in chutz laaretz should one put up mezuzas right away or wait till there is an obligation according to all on the 30th day of rent?
If one’s intention is to remain in the apartment for more than 30 days, the mezuzah should be affixed immediately upon entry.
The reason why an apartment outside Israel is exempt when renting for less than thirty days (Yoreh De’ah 286:22) is that it is not considered a diras keva. If a person rents for long-term, it is considered a diras keva from day one, and the obligation is immediately incumbent. This is the opinion of Chayei Adam, 15:22; Nachalas Tzvi to 286:22; Ben Ish Chai, year 2, Ki Tavo; and others.
Although this position could depend on a dispute between Rashi and Tosafos (Menachos 44a; see Pischei Teshuvah 286:18, based on Tosafos) concerning the status of a person until he has lived in a place for thirty days, one may follow this majority opinion.
This is also the ruling given by Rav Moshe Feinstein (Iggros Moshe, Yoreh De’ah 1:179).