My apologies, you are correct. From the research I’ve done, I haven’t found any references to being able to change the default expiry timer for multicast routing entries when using dense mode. In PIM-DM, traffic is pushed every 3 minutes, and routers downstream that don’t need the traffic prune it back. That’s the behavior of PIM-DM.
Now in order to optimize this behaviour, specifically for the pruning state of specific routes, there is what is called the State-Refresh mechanism which was introduced. Because every time the expiry timer elapses, there is flooding of large amounts of traffic into areas of the network that don’t need it. Any pruning that was enabled has expired and must be reestablished.
What state refresh does is it keeps the pruned state in dense mode from timing out by forwarding a control message down the distribution tree. This refreshes the prune state on the outgoing interfaces, resetting the expiry timer. The frequency of this refresh state must be less than the 3 minutes expiry timer.
More information about the state refresh mechanism can be found here which includes quite a clear description:
For examples of how it is configured, you can see this link as well:
I hope this has been helpful!