How long are films usually shown at the theater?

Especially if it’s a popular movie?

Yes, my boyfriend and I both love Twilight [hangs head]… and we want to see the movie together. He’s coming here December 28th, but New Moon is released November 20th. I don’t get to the movies much, so does anybody happen to know about how long it’ll be available, considering it will probably be a very popular ticket?