I have found that letting go of a novel is like having to let go of a loved one I have known all of my life and it is never easy. But I have grown wiser about recognizing that time with each book I have written and each life I have experienced.
As a novel is written, it ages. Sometimes, an author can write past an ending or multiple endings without realizing the story has grown old. An 800-page epic might be a young trilogy, or a 500-page novel may have become stale 100 pages ago. It is important to be able to recognize the difference as an experienced author or to have someone who can recognize it for you if you are just starting out.
I was one of those authors with a 500-plus page novel on his hands and an ending I was not happy with. It took the clever eyes of an experienced editor to see what I really had was the beginning of a series; the first book of which had really ended about 400 pages into the book.
A story can have a problem that needs to be solved, but a series will have many of them. A chapter, however, will have intermediate problems or dilemmas that cliff-hang from one chapter to the next or provide temporary safety. (Think of being safe in the trees from dinosaurs but not being out of the woods yet like in Jurassic Park.)
Before eReaders hit the market, back when I only read paperbacks and hardcovers, the best endings to books were the ones that left me feeling like I had just lost a best friend, yet their memories were still there for me to hold and re-live in the pages within. I didn't want the story to end, but I realized it had to.
So when does one know when it is time to move on to the next story, be that as a part of a series or as the end of a story that stands on its own? When there is nothing more to say? When you don't want to let go but realize you have to? When hanging on works to the detriment of all parties involved?
All children at some point must stand on their own. Our stories as authors are our children. We want to see them live long healthy lives, but the truth of the matter is all we can do is tell the story and some day let them go.
Is a story is ever completely finished? The end is where ever you say it is. The only advice I can give you is to choose wisely, and in the end, simply let go.