S.C. Code Ann. § 20-3-150

If the court awards the custody of the children to the spouse receiving alimony the court, by its decree, unless good cause to the contrary be shown, shall allocate any award for permanent alimony and support between the supported spouse and the children and upon the remarriage or continued cohabitation of the supported spouse the amount fixed in the decree for his or her support shall cease, and no further alimony payments may be required from the supporting spouse. For purposes of this section and unless otherwise agreed to in writing by the parties, "continued cohabitation" means the supported spouse resides with another person in a romantic relationship for a period of ninety or more consecutive days. The court may determine that a continued cohabitation exists if there is evidence that the supported spouse resides with another person in a romantic relationship for periods of less than ninety days and the two periodically separate in order to circumvent the ninety-day requirement.