California Code, Code of Civil Procedure - CCP § 329.5

The validity of an assessment or supplemental assessment against real property for public improvements, the proceedings for which are prescribed by the legislative body of any chartered city, shall not be contested in any action or proceeding unless the action or proceeding is commenced within 30 days after the assessment is levied, or such longer period as the legislative body may provide. Any appeal from a final judgment in such an action or proceeding shall be perfected within 30 days after the entry of judgment.