Cal. RTC § 8128

(a) Except as provided in subdivision (b) no refund under this article shall be approved by the board after three years from the last day of the month following the month for which the overpayment was made, or with respect to determinations made under Article 3, 4, or 5 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 that period. No credit shall be approved by the board after the expiration of that period unless a claim for credit is filed with the board within that period, or unless the credit relates to a period for which a waiver is given pursuant to Section 7676.

(b) A refund may be approved by the board for any period for which a waiver is given under Section 7676 if a claim therefor is filed with the board before the expiration of the period agreed upon.