California Code, Revenue and Taxation Code - RTC § 38602

(a) Except as provided in subdivision (b) no refund shall be approved by the board after three years from the last day of the month following the close of the quarterly period for which the overpayment was made, or, with respect to determinations made under Article 2, 3 or 4 of Chapter 5 of this part, after six months from the date the determinations become final, or after six months from the date of overpayment, whichever period expires the later, unless a claim therefor is filed with the board within such period. No credit shall be approved by the board after the expiration of such period unless a claim for credit is filed with the board within such period, or unless the credit relates to a period for which a waiver is given pursuant to Section 38419.

(b) A refund may be approved by the board for any period for which a waiver is given under Section 38419 if a claim therefor is filed with the board before the expiration of the period agreed upon.