I'd plan on an hour and a half. You may not be seated immediately. We waited about 10 minutes to get seated. It takes the Princesses about an hour to work the room, so you should see them all in the space of 60 minutes, but its possible that they will be a little delayed (or move a little faster). Our biggest wait was for the check at the end of the meal.
For one character breakfast, WE were done in 40 minutes, but we had one remaining character that we needed to see. It took another half an hour before she made it to our table.
Its likely you will get out in an hour, but if you are planning on meeting someone, plan for more. My husband and I met up after the PSB (I took my daughter, my son and DH rode Mission:Space a few times). I called him once we were out and took a few minutes to change her from her Princess Dress into shorts.