When do I put up the mezuzah?

Hi,

One is obligated to place mezuzot on a dwelling in which one will live for at least thirty days. However, there is some dispute as to whether the obligation begins immediately upon acquiring rights to the property, or only after one has lived there for thirty days.

Despite the dispute, one should place the mezuzah immediately, with a berachah, and not wait for the thirty days. The only exception would be a case in which one remains only temporarily, coming and going on the spur of the moment, like a bungalow in the country. [See more on this in the responsum.]

(Rav Moshe Feinstein, Igrot Moshe Yoreh Deah 1:179)

Have a great day,
Mordechai