California Code, Business and Professions Code - BPC § 5466

(a) Notwithstanding any other provision of law, as to an advertising display in place as of August 12, 2004, a cause of action for the erection or maintenance of an advertising display that violates this chapter or the laws of a local governmental entity shall not be brought by a private party against an advertising display that has been in continuous existence in its current location for a period of five years. However, if the advertising display has been illegally modified, the cause of action for the illegal modification may be brought by a private party if it is filed within five years of the date the modification was made.

(b) This section shall not apply to a cause of action brought by a governmental entity that is based on the erection or maintenance of an advertising display that violates this chapter or the laws of the governmental entity.