There is no retention requirement for complaint letters in general, but I
typically held these at least from exam to exam or three years, whichever was less. I
didn't find that I referred back to them. However, the regulatory agencies generally
keep the complaints they receive for 5 years.
If there is any litigation, you'd keep them longer according to the case. If they were
actually in your CRA Public File or subject to a specific regulation such as RESPA or
Reg. E, you would follow the retention requirements defined there as evidence of
compliance for those regulations.