Cal. RTC § 9152

(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 reporting period for which the overpayment was made, or, with respect to determinations made under Article 2 (commencing with Section 8776), Article 3 (commencing with Section 8801), or Article 4 (commencing with Section 8826) of Chapter 4, after six months from the date the determinations became final, or after six months from the date of overpayment, whichever period expires 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 claim relates to a period for which a waiver has been given pursuant to Section 8783.

(b) A refund may be approved by the board for any period for which a waiver is given under Section 8783 if a claim therefor is filed with the board before the expiration of the period agreed upon.