I'm not asking for you to give away the formulas of how much randomness this could involve, but I am curious at this point if my arena is stuck or if the amount of randomness in the arena building is far greater than I had thought.
I ordered a 50 seat increase in the lower tier seven days ago now. The estimated time on that was 6 days, so of course the fact that it's not complete now is slightly unfortunate but not unreasonable. However, in seven days, the estimated time has only dropped from 6 to 4 days, which means that not only am I not near getting one day's work per day, but I'm not even getting a third of the work done per day.
If it is possible that work can proceed that slowly (or even, with sufficiently bad luck, never progress) then I understand it's not a bug, but it's reached the level where I at least feel like it's time to ask about it.