I don't believe there is a standard, and I'm fairly certain there isn't a rule. No where in the UCC have I seen a rule that requires action within a specified time (as there is with regard to unauthorized EFTs in Regulation E, for example).
I believe that if your bank suspects its customer of being involved in the forgery scheme, you are entitled to refuse reimbursement and let the customer decide whether to pursue legal action. If the court finds in favor of the customer, however, you will have a deadline to comply.
I agree with John, if there is a rule that sets "service standards" on responding to a claim of forgery, it is not in the standard language of the UCC. Therefore, you are stuck with that law's general mandate to "be fair" and, believe it or not, that is enforceable. I encourage you to reach your decision as soon as you can and then tell the customer. For the purposes of self discipline, consider using 10 days as the outer marker for communicating your decision.
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