California Code, Revenue and Taxation Code - RTC § 30178

No refund or credit for amounts overpaid for the purchase of stamps or meter register settings shall be allowed or approved after three years from the due date for payment of the purchase for which the overpayment was made, or with respect to a determination made pursuant to this chapter, after six months from the date the determination becomes 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.