When is a screenplay finished?

I have been working on Where Wildflowers Grow and am wondering when it will be finished. I think the answer lies somewhere right after the director says cut and good for the final scene. I do not think that I am selling a finished product as much as a product good enough for a director or producer to see the movie. I wouldn't be surprised if a director asks for another scene to be written after film production begins. I have enoyed working on the screenplay and am excited to see it become a movie.