asks: "How do authors figure out when their book idea can be turned into a series, and if they choose to do a series, how do they decide how long it will be?"

The answer to this will differ by author. For me, since I am not a planner, it's usually by gut feeling. When I started writing Siena, I had no ideas about a series at all. I just had an idea about a girl with healing abilities, set in the faraway past. As I wrote, however, new ideas formed. And by the time I reached the end, I immediately began writing a followup story, because one of the other characters inspired me.

There are a few different kinds of series.

1. Each book in the series follows a large overarching plot. The Hunger Games and Divergent series both follow a character through her fight to overcome the establishment, and this grand journey is divided into multiple books. This type of series requires the most amount of planning, because you need to determine where the turning points are.

2. Each book is a separate story set in the same location/setting, with some overlapping characters, often from the point of view of a different character. My Forestfolk series is this kind. In my opinion, they are the easiest to form a series with, because you've already created a setting, but don't have to continue with the same character. It feels like a brand new story, because it mostly is. The risk of getting bored with the plot is minimal.

3. Each book continues with the same character in the next phase of that character's life. The first book in the Throne of Glass series follows the assassin's story through to resolution. The plot feels complete. The next book continues the same character's story in the next phase of her career, and is a completely new plot.

The question that was asked here, I believe, is talking about the first kind. You have this big, elaborate plot, and you don't know if it should be one book or several. Before you can determine this, you need to plan out the plot. Outline as much as you can so you can see how much material you have.

Stories have a natural ebb and flow, as well as multiple stages. A character must jump through certain "hoops" in order reach the end result. To span your story across several books, each book must have enough material to form an introduction to a new problem, a journey to overcome the problem, and a resolution.

In the Hunger Games trilogy, Book 1 saw Katniss volunteering for the games in place of her sister. She must survive the games, and she does. Book 2 saw a revolution sparked by Katniss and her actions. They create another competition for past winners, so again she must survive. And she does, plus more. The end of this one also sets the stage for Book 3. The final book then expands upon the revolution, the complications between Katniss and Peeta, and finally the resolution to the entire story line. Each book has an intro, a build-up, a pinnacle, and the finale, despite each book being a separate piece of the main plot.

One thing to remember when writing a series, regardless of which kind it is, is to try to finalize all the details of Book 1 before starting Book 2. Otherwise you will double your editing workload if you start changing things in Book 1.