Its a fairly large exhibit, we went 2 weeks ago. They let in a certain amount of people at a time and then i guess they let them get part way through and then let another group through. At the beginning there is a little "show" so I guess that is how they time the groups to go in. We went on a Thursday afternoon at about 2 or 3 pm and there were barely any people there. You can also purchase headsets to hear an audio tour as you go along, however we did not do that and didn't feel like we should have.
It is AWESOME!

Just as a warning though, you cannot take pictures and there are only three things you are allowed to touch. That really disappointed me! Also, they take your picture at the beginning of the exhibit which is kinda cool, but costs a lot to buy. Of course, there is a gift shop at the end with pricey but cool merchandise. One of my friends told me that most of the things are also at Universal's World of Harry Potter and surprising a bit cheaper (such as wands). There are things that say "Harry Potter Experience, NYC" if you so desire, and those obviously could only be purchased there.
We really enjoyed it and my girls are still talking about it 2 weeks later. I do suggest purchasing the "guidebook" they offer at the end since you can't take any pictures inside.
Hope you get to go, it's really cool!