We read that it can take hours of waiting to finally get to the top and the best time is between 8-10am. I would totally agree, we literally walked right up to the top without any waiting and walked through all the space where people would normally queue and it looked painful. Basically two floors of space designated to have people queue with information and facts scattered throughout.
We got there around 9:30, when we left at 10:45 (we did it that fast and spent over 45 min at the observation deck) the lines were starting to build quickly. Nightmare reviews said it took 4 hours to do what we did in just over an hour.
Get there before 9:30 so you do not waste half a day !
I would also recommend going on a sunny clear day. There were more clouds than i would have liked but it was still an amazing view. Nice piece of history as well.