Wikipedia
The word onah literally means "time period," or "season." In the context of the jewish laws of niddah, it usually refers to a day or a night. Each 24-hour day thus consists of two onot (plural for onah). The daytime onah begins at "netz hachamah" ( Hebrew for sunrise, also commonly called "netz") and ends at "shekiat hachamah" (Hebrew for sunset also commonly called "shekiah"). The night-time onah lasts from sunset until sunrise.
Marital relations are forbidden on an onah (as defined above) when a woman anticipates her menstrual cycle. This is called an "onat perishah," (time period of separation).
The term onah can also refer to the length of the menstrual cycle. Halachically, (according to Jewish law) one assumes that the "onah beinonit" (or average interval), is thirty days long.
The term "mitzvat onah" (a mitzvah performed at a set time period) refers to a husband's conjugal obligations toward his wife and is also used as a halachic euphemism for marital relations.
Some say: The mitzvah of onah – marital intimate relations – is defined in the Mishnah as fixed, regular times that a married couple may not be intimate with to together.
The times for conjugal duty prescribed in the Torah are: for men of independence, every day; for laborers, twice a week; for ass-drivers, once a week; for camel-drivers, once in thirty days; for sailors, once in six months. These are the rulings of R. Eliezer.
R. Joseph learnt: Her flesh implies close bodily contact, viz, that he must not treat her in the manner of the Persians who perform their conjugal duties in their clothes. This provides support for [a ruling of] R. Huna who laid down that a husband who said, ‘I will not [perform conjugal duties] unless she wears her clothes and I mine’, must divorce her and give her also her ketubah.
the Rabad enumerated four permitted "kavvanot" for sexual relations with rewards in the world to come: for procreation, for welfare of the fetus, for a wife’s desire, and that a man has desire to act promiscuously and relieves that through intercourse with his wife. Yet the last one is a lesser reward, since the man should have had the strength to resist. If He does not show any strength, and has sex anytime he wants, this would not be rewarded.
Maimonides and other rationalists saw sexuality and desire as an animal drive, not something for the rational man. He says: “The sense of touch which is a disgrace to us leads to indulge in eating and sensuality”, etc. There was a deep reaction to this about a generation after the Rambam. The growing movement of Kabbalah and other schools of the region had a negative reaction to this passage of Maimonides quoting Aristotle. the Nahmanides in "Iggeret Ha-kodesh" says: "But we who have the Torah and believe that God created all in his wisdom [do not believe he] created anything inherently ugly or unseemly. If we were to say that intercourse is repulsive, then we blaspheme God who made the genitals".