California Code, Revenue and Taxation Code - RTC § 16851

No refund shall be allowed or made after four years from the last day prescribed for filing the return or after one year from the date of the overpayment, whichever period expires the later, unless before the expiration of such period a claim therefor is filed by the taxpayer, or unless before the expiration of such period the Controller makes a refund. A claim for refund may be filed in such form as the Controller may prescribe, and the Controller shall allow or deny the claim, in whole or in part, and mail a notice of such determination to the claimant at the address stated on the claim.