Billy
When you write fiction, you start with character, the heartbeat of the novel. Characters are interesting people in terrible difficulties. Readers read books for stroy and for intimacy. They want to move into a character's life, live inside his thoughts and emotions, take on his goals and problems. So, fictional characters must be flawed and vulnerable. Bigger than life. So think about your favourite fictional characters and how large they loom within your imagination. Scrooge, Madame Bovary, Holden Caufield, Atticus Finch, Don Corleone, Lolita, nancy Drew, Jay Gatsby, Scarlett O hara, Harry Potter. All these characters have stature, presence, flair; they are memorable.
You start by knowing your characters as well as you know your own family. Create a biography, a sketch of an imaginary life with the traits and background important to the story you are telling.
The leading role is the person the reader most cares about. The story is about him or them and we want to know what happens to them at the end. Their motivations drive the story. As authors, we need to know everything about the lead character/s.
Having said this, I'm not sure I understand the crux of your question. In my eyes, you stop only when you reach the last word. How much of yourself do you slip in, well, that's up to you, the reader will never know. Most novelists start with their own life experiences. This means that they will use their personal lives and their careers as a potential place to draw ideas from. But, we can also draw from everyday experiences. Family. Friends. Colleagues. Neswpapers. Places. Dreams. So to answer your question, there is no stopping. You simply take what you can and write.
Hope this helps