California Code, Revenue and Taxation Code - RTC § 45652

(a) Except as provided in subdivision (b), no refund shall be approved by the board after three years from the due date of the payment for the period for which the overpayment was made, or, with respect to determinations made under Article 2 (commencing with Section 45201) of Chapter 3, after six months from the date the determinations have become 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 credit relates to a period for which a waiver is given pursuant to Section 45204.

(b) A refund may be approved by the board for any period for which a waiver is given under Section 45204 if a claim therefor is filed with the board before the expiration of the period agreed upon.

(c) Every claim for refund or credit shall be in writing and shall state the specific grounds upon which the claim is founded.