Other than the provisional credit requirement, there is no required timeframe for finalizing it since Reg E does not apply. What you can look at as a guide for your investigation is the timeframes that you, the card issuer, and the card aquirer have to act.
Once you file the chargeback the merchant has 45 days to represent the transaction if they are going to refute the chargeback. If the do not respond in this time frame, you're safe to finalize the credit.
If they do represent, then you examine the information and either side against the cardholder, for the cardholder, or possible hold off if you are going to file pre-arbitration to continue the dispute and don't have enoungh information to make a decision on the dispute.
You have 30 days from the date of the representment to file the pre-arbitration and the merchant has another 30 days to either accept or deny your request. If they deny it, you may then file arbitration with VISA and VISA will make a final determination.
When dealing with fraud, as you point out, Zero Liability covers the cardholder even if you cannot recover the funds. However, on a non-Reg E case, you may use all of the tools listed above to help make a decision with worrying about a 90 day time limit to complete your investigation.
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