When you're reading a series, do you prefer to know how many books there will be, or do you like it to be open-ended? For example, would you be more inclined to read a book if it were Book One of a trilogy, or Book One in an ongoing series?
The original Harry Potter series is seven books long. I don't recall J.K. Rowling promising early on that the series would end at the seventh book -- but that was implicit in the grade levels at Hogswarts. We had a sense that Harry's journey would begin and end with his Hogwarts education. On the other hand, B. V. Larson's success "Undying Mercenaries" series starts with "Steel World" and goes on for another 19 books. As a reader, do you prefer to invest yourself in a world that can continue, or to be told a story that you know will end in a timely manner?
Speaking for myself, I like to know there's an end before I begin. I don't particularly mind how far away the finish line is, I just want to know it's there: that the author is taking me somewhere specific. Let me know what you think!