An old interview with J.K. Rowling has come to light where she mentions the final Harry Potter book. You can watch the video
here or read the transcript below.
Warning: This may be a hint of things to come - so read at your own risk.
Quote:
My first and middle name is Joanne Kathleen. Kathleen is from my Dad's Mum, one of my grandparents. I was really lucky because the other grandmother was called Freda. So I think I lucked out actually getting Kathleen there somehow.
How many books am I planning to write? Harry Potter books? There will be seven in total, one for each... oh you look happy. I'm so glad. That would be very depressing if you went "oh no. Can't you give it a rest now." He's going to be at Hogwarts for seven years. In the seventh book he'll turn seventeen, so he'll come... In the wizarding... In my wizarding world you come of age at seventeen. Ok, so in book seven he'll be allowed to use magic outside school. (Jo laughs) So Dudley's really going to be in trouble. (audience laughs, Jo smirks) Now looking at you all being nasty to Dudley.
I get letters from parents saying I'm mean because I make Harry be nasty to Dudley. And I'm like, "Have you read Dudley? Do you understand what Dudley's like?" (audience laughs, Jo rolls her eyes) I mean there's turning the other cheek and there's just being a moron.
Source:
Dissendium