We did BBB several times in DD's younger years. We typically would have the BBB reservation at 10:00 or 10:30 with a CRT lunch reservation at noon. We would get to the park at opening, do a ride or two while lines were short and then head to the BBB. 10:00 was really best because we wouldn't stress out about making the CRT ADR if BBB was running late. It also worked well because if DD wanted to take the hair down, we would have CRT lunch first with princess pictures and sometimes we would go track down a couple of princesses we didn't see at lunch right after lunch before the hair came down.
I've heard the buns are not nearly as tight as they used to be. DD had the bun once back in the days when they were really tight and was begging to get it out within 2 hours- it barely made it through CRT lunch. There was one other hairstyle she had there that she wanted out before the end of the day-something with tight braids- I don't think it exists anymore. There were other BBB hairstyles that she didn't have a problem with and left in all day, but she always wanted the hair down and washed at the end of the day- she was never a sleep in it and wear it a 2nd day kid. Having a morning BBB appointment worked for us because she had the hairstyle all day (if it wasn't too tight and she wanted it out) for photos.