I’m having difficulty finding an AST bus schedule for Catania to Rifugio Sapienza. I know the bus leaves at 8:15 AM, and returns at 4:30 PM, but there is conflicting information as to whether it operates in November.
You can check the schedule for your exact travel date here. I only found an entry for "Etna", rather htan "Rifugio Sapienca" in the list of destinations, but the resulting departure time was 8:15, so I think it's the right bus.