Streets and Highways Code § 9187
CHAPTER 5. Consent of Landowners to Refunding and Reassessment
The validity, sufficiency, or genuineness of any of the consents, or the finding and determination of the legislative body thereon, shall not be contested in any action or proceeding unless commenced within three months after the determination of the legislative body has been made. Thereafter no person in any action or proceeding may plead or prove that said consents, or any thereof, or the finding and determination of the legislative body thereon, was in any way defective, invalid or insufficient.