Cal. Rev. & Tax. Code § 13104

If the board fails to mail notice of its action on a claim for refund or credit within six months after the claim is filed with the board, the claimant may, prior to mailing of notice by the board of its action on the claim, consider the claim disallowed and bring an action against the board on the grounds set forth in the claim for the recovery of the whole or any part of the amount claimed as an overpayment.