Since these are paper items (or started out that way), they are covered by the UCC. If both banks are in Pennsylvania, you'll look at section 4-406 in the PA UCC, which provides a one-year statute of limitations.
There is also the possibility that the paying bank has in its deposit contract a shorter time period for its depositors to claim items are unauthorized. That may work in your favor, since you have a defense against a claim from the paying bank if the paying bank's customer's claim wasn't timely.
I do hope you have a contract provision with the debt collector that you have the right to pass on to the debt collector any enforceable claim that a RCC was unauthorized.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8