It depends on who you are trying to appease. The simplest argument is that you shouldn't have charged it so you should go back to the date the problem started. If you are concerned about a regulator, my experience has been that they don't typically expect file searches to go past five years but they also consider whether documentation exists to actually conduct the search. You could also try reaching out to your local office. Lastly, there is the shorter TILA statute of limitations.
My view is that unless its wildly complicated to do, might as well make everyone whole. If you have compliance exams it is also a nice way to demonstrate your commitment to corrective action.
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Opinions expressed are my own and do not reflect legal advice or the opinions of my employer.