When the ride gets delayed you will get the additional time/scene in the one pre-show. It isn't actually affected by what people do or don't do within the room (I don't normally move around and often stand to the side if there is someone younger behind me so that they can see).
There are 2 preshow rooms/videos to go through. I'm not certain as to the length of each but from personal experience and others posted on here, 30-45 minutes seems to be a norm for total time from scanning FP to getting off the ride (unless there's a mechanical delay).
It will depend on how long the standby line is, and during Christmas week, it will likely be long. We rode it in October which is moderately busy. It took us roughly 15 minutes to get from entering the FP queue to entering the first pre-show room. Between the two pre-show rooms, probably another 15 minutes. So by the time we were done, about 30-40 minutes in total. But like PPs have mentioned, you're not standing in a queue during that time. The pre-shows break that up.
And honestly, even if it is 51 minutes, its worth it if you haven't ridden it yet.