The studios don't remove films from theater locations (well, until they decide to completely take them out of circulation) - they want each and every one of their films to stay in each and every theater they're in for as long as possible. But a whole slew of factors affect how long any individual film may remain at any given theater - and the studios have little control over the vast majority of them. But chief among the factors is that theaters have already made commitments to show upcoming releases - and the theaters can't simply say to a studio, "We know we committed to showing your film, but surprise, we're not going to!" - well, not unless they want to be sued for breach of contract.
They can, however, hold onto a film for as long as possible if they feel they have room for it - and that's why many times two films will share screens and divide showings between them. And the fact that DS is in a position currently where it is sharing screens in many theaters is actually a good thing because in many cases the theaters could have just as easily dropped DS completely in favor of showing a newer film in its place.
32 days 20 hours 12 minutes 48 seconds since the Depp/Burton Dark Shadows has been in release(ET)!!