This may be a state issue.
In ours, the only guidance I see is that Audit Reports presented to the Board must be kept for 3 years after preparation (New Mexico).
Personally, I'd keep them at a minimum until the next regulatory examination and, if no guidance can be found, set a limit such as for Audit Reports.
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I hear and I forget. I see and I remember. I do and I understand.--Confucius