Hello Tracye and thanks for bringing such a great question to the Disney Parks Moms Panel. 
 When my own Princesses were much younger they adored dining within the fairytale setting and elegant ambience of 
Cinderella Castle set in the heart of the
 Magic Kingdom Park and I'm sure if I asked them right now they'd both dine there again in a
 Bibbidi Bobbidi Boo! 
 In my personal experience I would set aside at least 2 hours or more from when your Advanced Dining Reservation time begins for either lunch or dinner at 
Cinderella's Royal Table.  
 This ample slot should allow most Guests to be comfortably seated, dine on delicious food, enjoy meeting with Disney Characters plus time to soak up the whole enchanted experience - after all, you wouldn't want 
the last stroke of midnight to chime before you've quite finished so to speak! 
 Not to mention extra time for capturing lots of perfectly, perfect 
Disney's PhotoPass Service pictures and creating many magical memories which are sure to last an entire lifetime! 
 Have a great Disney day. 
 ~ Jane