Cal. Water Code § 71946

Any action or proceeding in which the validity of any bonds or of the proceedings in relation thereto (including the formation of an improvement district for which bonds are authorized to be issued by the voters thereof at a bond election) is contested, questioned, or denied shall be commenced within three months from the date of the bond election; otherwise the bonds and all proceedings in relation thereto (including the formation of the improvement district) shall be held to be valid and in every respect legal and incontestable.