I am seriously doubting 6 hours.  I would think that once the line got up to 3-4 hours, most people wouldn't be willing to get in line.  Even if I was someone who paid thousands of dollars to come over from Europe, if I saw that I would have to waste a whole day in line for that one ride, I certainly wouldn't do that.  Maybe Potter fanatics, but not your general public.  It also sounds like it's a line that steadily moves, not the kind where you stand there for 10 minutes and then finally move a little bit like the Simpsons.