I have not had a muster drill while the ship was sailing out of port. Granted, I have only two Carnival cruises under my belt but I have quite a few on other cruise lines. I have had three drills "after" we have set sail. These have been on Tahitian, Mediterranean and China cruises. These were about an hour after sailing or the second day when we had done an overnight on the first day.
My biggest complaint about muster drills are people not showing up, taking their time to get there or talking while the crew is going through their instructions. This just makes everything take longer.