I think the wait times are almost always going to be long. I've actually stood in line for the PFF twice. Once at night in the last hour before it closed and once in the morning before it opened. The wait at night was much longer, I'm thinking around 40 minutes. In the morning, I showed up about 15 minutes or so before PFF opened and didn't have to wait long once they took everyone over into the queue.
The tradeoff with doing it early in the morning is losing that valuable early morning touring time. I think you have to assess just how much your family wants to see the princesses. And be aware that if it's just one certain princess you want to meet, you may have to wait in long lines multiple times hoping, but never guaranteed to see her.
Generally speaking, you'll get a couple of minutes or so with each character. It's more than just smile, say cheese, and move along, but it's not like you'll be spending 5 minutes discussing the finer points of War and Peace with Belle. That said the characters can do a lot in those 1-2 minutes to make the interaction memorable and once in a while you can have more interaction time.