Every 24 hours, the Centralized Cleanup Service runs and deletes indexes that are older than the number of days set for Retain index for n days on the MediaAgent Properties > Catalog tab.
For example, if Retain index for n days is set to 60 days, the Centralized Cleanup Service deletes indexes that are more than 60 days old.
The scheduled cleanup deletes indexes for any subclient that does not have active, running jobs, including de-configured and hard-deleted subclients. This way, index data is pruned for subclients whose data has been aged (see Data Aging: Overview) and can no longer be browsed.