Cal. Pub. Util. Code § 33016

Any action or proceeding, other than a petition for election pursuant to Section 33002.2, which contests, questions, or denies the validity or legality of the formation of any benefit district or zone, the issuance of any bonds therefor pursuant to this chapter, or any proceedings relating thereto, shall be commenced within six months from the date of the formation; otherwise, the formation of the benefit district or zone, the issuance of the bonds, and all proceedings relating thereto shall be held to be in every respect valid, legal, and incontestable.