InterMapper Data retention policy setting has been fixed in the new 5.3.2 release.
InterMapper Data Center is a facility store and archive data collected by InterMapper. Although InterMapper stores data efficiently into a build-in or a external Postgres SQL database, if you have significant amount of devices and data collected, letting the data store growth indefinitely may eventually fill up your database and may cause service interruption for other applications running in the same server.
When you arrived at the Data Retnetion Policies setting page of InterMapper DataCenter, upon clicking "Save Changes". If you receive a generic error page, this is a known bug in InterMapper 5.3.1. Upgrade to InterMapper 5.3.2 and this issue should be resolved. You should receive "Data retention policies saved" message as shown above.