Cal. Pub. Util. Code § 30970

Any action or proceeding, wherein the validity of the formation of the improvement district or districts or of any bonds or of the proceedings in relation thereto is contested, questioned, or denied, shall be commenced within three months from the date of the election; otherwise, the bonds and all proceedings in relation thereto, including the formation of the improvement district or districts, is valid and in every respect legal and incontestable.