Ketubot, Chapter Five, Mishnah Six
This mishnah teaches that a husband has an obligation to have sexual relations with his wife. How frequently he is obligated depends on his job.
The idea that a husband has an obligation to periodically have relations with his wife is derived from Exodus 21:10 which states that if a man takes a second wife he cannot diminish from her three things: food, clothing or conjugal rights. With regard to our issue, if a man has two or more wives he must provide each one with their conjugal rights. You can imagine that this might have been one deterrent to the practice of polygyny (the proper term for the practice of men marrying more than one woman).
1) A man forbade himself by vow from having intercourse with his wife:
a) Beth Shammai says: two weeks;
b) Beth Hillel says: one week.
2) Students may go away to study Torah, without the permission [of their wives for a period of] thirty days; workers for one week.
3) The times for conjugal duty prescribed in the torah are:
a) For independent men, every day;
b) For workers, twice a week;
c) For donkey-drivers, once a week;
d) For camel-drivers, once in thirty days;
e) For sailors, once in six months.
f) These are the words of Rabbi Eliezer.
Section one: A man cannot make a vow to forbid upon his wife anything which he is mandated to give her by law. The man in this mishnah, perhaps in a fit of anger, forbade his wife from have sexual relations with him. This is not permitted and if he does not have his vow annulled (a process we will discuss in tractate Nedarim), he must divorce her and pay her the ketubah. However, he is not obligated to divorce her that very day; rather he is given a period to cool off and hopefully have his vow annulled. According to Beth Shammai he is given two weeks and according to Beth Hillel he is given only one week. After that length of time, he must divorce her and pay her the ketubah.
Section two: Since a husband must have relations with his wife, he cannot be away from her for a long period of time. According to this section, a Torah scholar cannot leave his wife without her permission for longer than thirty days. A worker can be out of town for only one week. If either wish to remain away from their wives for a longer period of time, they must receive permission.
Section three: This section delineates how often in general a husband must be available to have relations with his wife. The frequency depends on his occupation. An independent man, meaning one who doesnt work, must have relations with his wife every day. Note that this does not mean that he actually has to do so, but rather that if she so desires, he is obligated. He cannot claim that he is too busy to have sex with her. Workers must be available twice a week. The Talmud explains that this refers to workers who work in the city; those who work outside the city are obligated only once a week, as we learned in the previous clause. Donkey-drivers, who travel short distances must be available once a week. Camel-drivers who travel longer distances must be available once a month and finally, sailors who travel for long periods of time, need to return home once every six months. This all refers to situations where the wife has not given her husband permission. If she has given him permission, he may stay away longer. She might give him permission if, for instance, for the sake of supporting the family, the husband had to be away for a long period of time. However, it is her right to demand that he find work closer to home.