We need a reason for writing the book, which is the problem, a plot, and an ending whether it be happy or sad. We need a hook to grab the reader and agent. We need to end when we feel the story is covered from the beginning with each character and there is nothing left to tell.
Always put the book aside for a week or longer and go over it reading each line over to see if a problem was left unsolved. Did a character end in the story with their future of did you leave them hanging.
Don’t rush a book to get it published. Take your time. Make sure the story doesn’t jump all over pages. Look at each character and see how their lives end.