It is my understanding it takes 20 min or so, but the later in the day it is, the more backed up they can become, especially at MK.
My BBB strategy is to have an early am appt. and then we booked a CRT lunch. This gives us plenty of time for makeover, photos (will prob skip) and most importantly: we will be ready for Fantasyland and that elusive first ride on Dumbo!!
It actually took some perserverance and several stalker-like phone calls to get an early am BBB appt to coordinate w/our CRT lunch. I was considering breakfast but I didn't want to spend a morning in the park and not have time for a ride! The princesses also come out at lunch and by then, we will be glad for the break.
There are some cute BBB makeovers on youtube for research purposes!! Happy planning!