My girls had their appointment at 8:30 am MK's BBB. They waited 5 min to be seated and then it took about 30-40 minutes (my 4 year old's FGIT had a little difficulty putting her hair up in the pony tail to be ready for the bun).
When we went to Expo hall for the portrait session, there were 2 groups ahead of us and we had to wait for them to finish each session takes about 10 minutes if you have 2 kids. They do a pose standing, a pose sitting on the chaise, and then if there is a sibling, they pose together.