With all the publicity, a big opening was assured. The big indicator for this (and nearly any film) is the second weekend drop-off. A drop-off that is considered average is around 30-40%...this means that while the first week crowds are satisfied, word-of-mouth is good, and people that missed it first week are going out to see it.
Often, though, blockbusters and not-so-great films will drop 50-70% on week 2, because A) these films open on so many screens, everyone that wants to see it in week one gets to, and B) word-of-mouth is terrible.
A drop-off of 5-15% is very rare, this usually means that the movie's a sleeper hit, namely that it wasn't advertised well, but was so good that people came back and brought their friends.
F9/11 has three things working in its favor here. This week's per-screen average is high, meaning most theatres are selling out. This in turn means that a ton of people didn't get to see it when they wanted to, so they'll come back next weekend. Also, since it only opened on 800 screens (as opposed to 3000 for a typical studio release), there's a strong chance more screens will be added next week, meaning a bump in the gross. Finally, unlike most blockbusters, there's really no competition for the audience. Spiderman isn't going to take a ton of business from F9/11, because they're such different films.
The downside is that since this opened so big, and had such a high want-to-see from the general public, that people rushed to see the film. This usually happens with sequels, and spells a big drop for week two.
For this to really be considered a hit, and not a one week wonder, F9/11 needs to drop no more than 40%. Assuming the film hits $22 million this weekend (and it could be higher if Sunday's numbers are big), that means we need to do $13 million next weekend. With Spiderman hogging the big auditoriums next week, this is going to be rough. Drag your friends back, this one needs staying power.