Real estate signs advertising subdivisions of land and projects requiring site plan approval which have received preliminary or final approval shall be limited to one sign per entrance to the subdivision or site provided that no such sign shall exceed 24 square feet in area, nor be located closer than 20 feet to any street line. Such sign permits shall be valid for a period of six months and then such signs shall be removed, unless the permit is renewed. Such signs shall be removed in the case of land subdivisions, when 75% of the lots created have been sold and in the case of projects receiving site plan approval, upon the issuance of a certificate of occupancy.