Ketubot 62 - Man's obligations to his wife
A husband has to provide his wife with food, clothing and intimacy . If he quarrels with her, and in his rage vows to never have relations with her, this vow does not take effect, because he cannot take away her right. However, if he phrases his vow as "the pleasure of relations with you is prohibited to me," then he has created a prohibition for himself. Since now he is not a good husband, he should divorce her immediately and - because it is his fault - pay her the complete amount of her Ketubah.
If, however, he limits his abstinence vow for a week or two, then we need to estimate when this situation becomes unbearable, and he has to give a divorce after that. How long is that? Beit Shammai say that it is two weeks. What is there logic? - In the Torah there is a situation where they have to be apart for two weeks, after the birth of a daughter, but more than this is too much. Beit Shammai compare the situation caused by the husband (vow) to the birth, also caused by the husband. However, Beit Hillel argue that "too long" is one week. They compare this to niddah , which lasts for one week, and make an analogy between common occurrences - niddah and vows.
Practically, however, people with means and without worries should be intimate with their wives every day, laborers working in town - twice a week, donkey drivers who travel - once a week, and sailors, who are often absent for a long voyage - once every six months.
Art: Awaiting the Sailors Return by David Woodlock