Before it leaves a multicast group, a receiver sends an IGMP leave message to the DR. If all directly connected members of a multicast group leave or time out, and no downstream members remain, the DR sends a prune message upstream and PIM-SM deletes the route entry after that entry times out.
When the system ages PIM mroutes, it does not clear the (S,G) entry for an inactive route immediately after the expiration period. Topology and hardware conditions can affect the polling interval and cause an inactive route to remain for up to 12-15 minutes.