CAL. CIV. CODE § 1747.40 : California Code - Section 1747.40

If a card issuer fails to give a timely response to an inquiry of a cardholder concerning any debit or credit applicable to an obligation incurred through the use of a credit card, he shall not be entitled to interest, finance charges, service charges, or any other charges thereon, from the date of mailing of the inquiry to date of mailing of the response.