If you have started writing a book without planning then you may face this dilemma- when do you finish your book? What's the number of chapters you need for a credible book? There is no correct answer to this, it all depends on your plot and where its going. If the events in your plot have slowed down and you're running out of ideas, that may signal that the end is near. I would recommend planning the last five chapters of your book in detail, making sure you tie up any plot holes that you left in your book.

Finishing a book is always hard, but what is harder is deciding whether to leave the book as a one off or making a sequel/prequel. If you have an idea for a sequel/prequel then I suggest writing that but always listen to your readers. What I did with books I wrote was ask my readers whether they wanted a sequel and sometimes they would say yes and sometimes they would say no.

These decisions are always personal ones to make and there's not much advice I can give other than go with your gut instinct.