It's going to do really well. It's going to be one of the top grossing movies of the year, definitely top ten and probably top 5, but it's going to be less than The Dark Knight and nothing this year is going to come close to The Avengers in box office results, not Twilight, not The Hobbit, not Brave, nothing.
The movie is on pace to be the 3rd highest grossing movie of all time. Even a huge blockbuster like The Hunger Games was something it passed in just a few weeks (both worldwide and in the US). The Avengers was a movie that did insane numbers that you don't see very often.
Meanwhile, despite this being the finale for Nolan and company, as others have said The Dark Knight had The Joker who is nearly as popular of a character as Batman, and it had the big controversy of Ledger dying before it came out. Those two things drove the movie to a much higher level than it would have been otherwise (especially the use of The Joker), while this movie doesn't have either of those things. It will do really really well, but not Dark Knight levels, and certainly not Avengers levels.