In Hilchot Ishut the Rambam gives different requirements to men to fulfill their conjugal rights obligations (http://www.chabad.org/library/article_cdo/aid/952888/jewish/Ishut-Chapter-Fourteen.htm).
- Healthy men who are pampered and indulged - every night
- a talmid chacham - once a week
- a worker - twice a weak
- a seaman - once every six months
The shulchan aruch gives the same halachah.
What is the motivation behind these numbers? Is this the lower limit, the upper limit? If marital relations is a holy act, why should it be limited? If marital relations is negative, why should the pampered and indulged men have it every night?