I rushed the show last Thursday and was that person who gets there waaaay too early (before 6am), which made me first in line. The next people didn't get there till after 7am (maybe 7:15-7:30?), and after them the line formed at a pretty steady pace. I couldn't say how many people were there by the time the box office opened, but it was a lot (30 seems about right). I will say that I'm not sure how many of them actually got tickets, because even as first in line I only got the two seats on the very end of row J, so I'm not sure what else they even had available for rush tickets. I do know the group right behind me, who were getting four tickets total, ended up in one of the boxes.
Hope this helps a bit.