I just looked it up, this is what wiki said:
Pon farr (English pronunciation: /ˌpɒn ˈfɑr/) is a term used in the fictional Star Trek canonical TV series. In Star Trek, pon farr is a psychophysical condition affecting Vulcans, in which Vulcan males and females go into heat every seven years, going into a blood fever, becoming violent, and finally dying if they do not mate with someone with whom they are empathically bonded or engage in the ritual battle known as kal-if-fee. Vulcans also mate outside of Pon Farr, and with species other than Vulcan. A common misconception associated with the series (and Spock in particular) is that Vulcans only have sex once every seven years. However, pon farr is not parallel to the sex lives of Vulcans, and they are able to have intercourse without the affliction, and thus more than once every seven years.
in the ENT episode "bounty" T'Pol goes in to pon farr. She talks about it and i don't remember what she said exactly but you should watch it.... It's a good one.