Maintaining External Suppression Files

A question about names mailed from an outside list:

Does your organization have a standard for how long you keep names on your external suppression file (i.e. names acquired from an outside list for a mailing that then called afterward to say "do not mail me").  We add those names to a list and suppress them in every future mailing.  However, We have found our list gets quite long year-by-year and do we need to continue to keep names on this file from say, 2001 (particular in an urban transient population)?  Any suggestions or insight as to how your organizations handles this would be quite helpful. 

Thank you,

Chuck Buchanan

Manager, Direct Marketing and List Management

92nd Street Y