I think you know when you reach the last chapter. Like any proper story there is a climax then a steady or maybe abrupt decline, but at the last chapter you know the story is going to end. It should not be a surprise. It may be for others but you are the writer; you control the outcome of your story. If you are unhappy make a change. If that does not work then end it. Do not drag it out longer than necessary. It will only hurt more in the end.