How do I purge Sales activities?

Old activities are purged automatically by the Activities Purge process, which runs automatically every week. You can't change the frequency of the process, but you can edit the criteria the process uses to select which activities are purged as described in the topic Edit Criteria for Populating and Purging Activities in Individual Records

When a record is deleted, the process automatically purges all the corresponding activities. The activities that are removed by the process can't be retrieved.

Activities that get tracked in each record include email exchanges, notes, call logs, past and future tasks, and appointments, and recent updates to the record itself. The application tracks these activities on accounts, opportunities, contacts, and leads.