In response to above, in a recent interview with Time, she stated the next book is definetely the last, unless Rowling turns out to be a liar (which I doubt.) She also said it will be her last fantasy book/series but that she does plan to continue writing.
It probably will take a couple years for the last one to come out too
she said that she only had a baby of a book so far and probably won't really start it until next year. I suspect the movies are keeping her busy, I would rather her concentrate on the books before bothering with the movies, but oh well.