A system and method are provided for dynamically adjusting advertisement (“ad”) time for a multicast channel. In one embodiment, a variable ad insertion function operates to receive one or more multicasts of media content from a core distribution network, insert ad timeslots into the media content, and optionally perform late-binding of ads to the ad timeslots. The media content including the inserted ad timeslots and late-binded ads, if any, is then multicast to associated client devices over a feeder network using corresponding multicast channels. For each multicast channel, insertion of ad timeslots into the associated media content is dynamically controlled based on the number of users currently consuming or predicted to consume the multicast channel, the number of client devices receiving or predicted to receive the multicast channel, or both.