I mean, I could see this going on for many years without a problem, and when Bale reaches a proper age, we are actually wandering into "The Dark Knight Returns" territory, which would be awesome.
The story I heard a couple of years ago was that after "The Dark Knight," Bale asked his lawyers to look at his contract and see if there was any way he could legitimately avoid
making a third Batman film, and they said, "Nope, you're stuck with a three-picture deal. If the third picture goes into production, you've got to be there!"
So once his contract has been fulfilled by three Batman films in a row, I do believe Bale will move on to other things, and someone else will need to be Batman in any subsequent film.