Retention / Purge Process

How and when are past events purged? How can this period be adjusted? Specifically, we have singular events which are aperiodic for scheduling, and when another such event is planned we want to go back to the original event and reschedule it. However, the original event seems to have disappeared.

I have searched the docs and not found anything. When searching the web AI indicates there is a "retention period/purge period" parameter in Configuration I but cannot find it.

Thanks,
Paul