…STUDENTS MAY GO AWAY TO STUDY THE TORAH, WITHOUT THE PERMISSION [OF THEIR WIVES FOR A PERIOD OF] THIRTY DAYS…
— Babylonian Talmud, Ketubot 61b
This mishna lists the varying lengths of time a husband may separate himself from his wife, according to Rabbi Eliezer, for varying reasons. The accompanying discussion in the gemara cites two different proof texts to argue that one month of studying at a time — with or without a wife’s permission — was “usual”:
STUDENTS MAY GO AWAY TO STUDY etc. For how long [may they go away] with the permission [of their wives]? — For as long as they desire.
What should be the usual periods? — Rab said: One month at the college [literally: “here”] and one month at home; for it is said in the Scriptures, In any matter of the courses which came in and went out month by month throughout all the months of the year.* R. Johanan, however, said: One month at the college and two months at home; for it is said in the Scriptures, A month they were in Lebanon and two months at home.**
— Ketubot 62a
The discussion continues, fodder for another day’s consideration. One point behind the 30-day marriage/Torah rulings, meanwhile, is that relationships require committed periods of attention, or at least proximity. The text seems to assume that a period less than 30 days is negligible, or at least manageable, when it comes to marital absence. See this newly married rabbi’s thoughts on this passage.
— *I Chronicles 27 describes a set of month-on/month-off rotations for national service.
— **I Kings 5:28 says that King Solomon conscripted 30,000 to build the Temple. However, only 10,000 worked each month, while the other 2/3 of the workforce was home (with their wives): each man was then home for two out of every three months.
— citations are from Soncino (public domain) Talmud translation
As part of NaBloPoMo (National Blog Posting Month), a cousin of NaNoWriMo (National Novel Writing Month), “A Song Every Day” plans thirty daily posts with some connection to the number 30.