The way the procedures work now, at park opening the line for FoP is being sent up the path towards Harambe, rather than filling in the internal queue. (Quite odd, but it is what it is.) When we got to the park on the one day we rode standby, we were about 30-40 yards up that path - not counting the part snaking through the area right in front of the attraction. I think this was at about 0840, shortly after we were let in to the Pandora area. From *that* point up on the path, our wait was about 35 minutes. If you see the line extending a lot farther back up the path, that suggests your wait will be correspondingly longer. The line moves nominally 200 people every eight minutes.
CAVEAT - this was in mid-September, before the fast-pass mix was changed from 50-50 to 75-25. So this may mean the standby line will move slower than my experience.
However, one thing I feel safe in saying is that the posted time should not be the thing that makes your choice for you. At park opening, things are changing rapidly and that number is not a lot more than a guess.