California Revenue And Taxation Code Section 11339

(a) Any assessment made outside of the regular assessment period may be the subject of a petition for reassessment. A petition for reassessment may be filed on or before the 50th day following the date of the notice of assessment. (b) The board may extend the deadline for filing a petition for a period not to exceed 15 days, provided a written request for the extension is filed with the board on or before the expiration of the period for which the extension may be granted. (c) If a petition for reassessment is not timely filed, the assessment of the board shall be final. The board may consider a petition which is not timely filed to be a claim for refund. (d) The board shall hear the applicant on the petition within 90 days of the date on which the petition was filed.