Is this the End?
Every story has to end. The most important thing that has to happen before a story ends is that the central conflict of the story has to be settled. The protagonist wins. The protagonist loses. The protagonist realizes that she has both won and lost. Whatever the case, the crisis is settled. What then?
Say a Little or Say a Lot?
In movies, you frequently see them end the story at the moment, the very moment, when the central conflict has been settled. Sports movies are famous for this. The Karate Kid ends just after Daniel has defeated his nemesis Johnny to win the karate championship. He is literally still standing there with his arms in the air as his instructor Miyagi looks on with pride. There is no denouement whatsoever. It ends at the moment of triumph.
On the other end of the scale you have the Lord of the Rings Trilogy (both the books and the movies). It can be argued that half the final book (and movie) are denouement. We see how the conflict has changed each of the central characters and we follow them as they return to their former lives or find that they cannot return to their former lives. The World According to Garp (the book, I never saw the movie) actually takes the time to follow each of their characters all the way to their various eventual deaths. It tells you how their lives played out in the aftermath of the central crisis.
All of these choices are valid, but there are definitely consequences to each choice. A brief, or nonexistent, denouement runs the risk of the reader not really feeling that the central conflict had a significant effect on the characters. They may end up feeling as if their time has been wasted or feel that the characters haven’t really changed. An especially long denouement, by contrast, runs the risk of leaving the reader bored. Once the tension of the crisis has been released, the reader knows that the conclusion is coming. The longer you take with the denouement, the longer you will have to keep the reader’s attention without having the tension of the conflict to keep them invested.
Be Fair to your Readers
One of the most controversial denouements is the end of the Harry Potter series of books. Because the series lasted seven books, the readers were invested in many, many characters. People wanted to know how all of these characters turned out. What readers got was a twenty page denouement, set years later, that answered very few of the lingering questions. This upset most readers — quite understandably. When you spend several thousand pages discussing the lives of a set of characters, you should expect that the readers will be invested in the outcomes for each of these characters that they have grown to love over the years.
My simple advice is that a denouement should last long enough for the reader to feel satisfied, but no so long that the reader gets bored. Make sure that the central themes of your novel get at least a moment of reflection in the denouement and that your readers are clear about how the novel has changed your characters.