Hairspray is not an event movie, like the harry potters, die hards, or transformers. most people will not be breaking down the doors to see this one. I will also be shocked if it makes over 20 mil the first weekend, but I do think that if the movie is enjoyable, with good word of mouth, people will trickle in through the next few weeks when they find the time.
If your looking for it to break some records or something, go look someplace else.
"I'm an American, Damnit!!! And if it's three things I don't believe in, it's quitting and math."