From CT3

In dense mode operation, routers initially flood multicast traffic for all groups out all multicast-enabled interfaces. Routers which determine they have no clients interested in receiving the traffic then send prune messages up toward the source, requesting that the flow of multicast traffic downstream be stemmed. This process repeats toward the source router until all extraneous branches have been stemmed.

This process of flooding and pruning repeats every three minutes. Because of this and other inefficiencies, PIM-DM isn't often recommended for real-world deployment.