Lynn Bierma, interlibrary loan coordinator for the Illinois State Library advises:
CONTU (National Commission on New Technological Uses of Copyright Works) specifies filled requests of articles should be retained for the current year, plus the previous three (3) years’ records. Electronic request records should be purged on the same schedule. For filled requests for books, the rule of thumb is the same.
The requesting entity shall maintain records of all requests made by it for copies or phonorecords of any materials to which these guidelines apply and shall maintain records of the fulfillment of such requests, which records shall be retained until the end of the third complete calendar year after the end of the calendar year in which the respective request shall have been made. [Final Report of the National Commission on New Technology Uses of Copyrighted Works, Chapter 4 – Machine Reproduction – Photocopying, CONTU Guidelines on Photocopying under Interlibrary Loan Arrangements, http://digital-law-online.info/CONTU/contu24.html#sec7]
Your library, if associated with a company, or state, city, or federal institution, should also be aware of the record retention policies of its governing body.