I would say no later than 30 minutes before showtime. Preferably you should aim for 1 hr-45 minutes to have your choice of spots. That whole area is also very congested before Fantasmic with everyone trying to get into their spots, so you don't want to leave it til the last minute. But anywhere in the BB viewing area is good. If it is only BB in the area, then there is plenty of room to spread out. But, if it is a super crowded night and the other dining packages areas (River Belle, Hungry Bear) fill up early the CMs sometimes start sending any last-minute guests into BB, so you don't want to get stuck behind all of that!