How much did J.K. plan when she first started writing?

I have often wondered how far J.K. had gotten in developing the entire seven-book arc when she first sat down to write The Philosopher's Stone. It's sort of the same argument people have about George Lucas and the Star Wars movies: the author says the entire series was planned from the start but it seems pretty hard to believe.

And even if you do believe that the creator had some concept of the larger project when he or she sat down to pen the first installment, it's really hard to convince yourself that the creator had planned the entire extended story in detail before embarking on the first book.

I'll admit it, when it comes to this discussion, I'm a strong skeptic, with regard to both Lucas and Rowling. I am willing to accept that, for example, J.K. dreamt, as she began The Philosopher's Stone, of writing a series of novels but I doubt she got much further than that.

Certainly, the concept of Hogwarts being a seven-year school isn't strongly presented in that first book. Harry is simply there in his first year, interacting with older students in their upper years.

On the other hand, it is clear from the ending of the first book that Rowling anticipated writing more and that the Potter/Voldemort conflict would be at the core of the following novels. I'm just not sure she had the following six books outlined in detail in her mind at that point.

Maybe I needed to read The Philospher's Stone again, in English this time, and make note of all the examples that suggest she did plan the remainder of the books and the examples that undermine that claim.