How much of any given novel is truly "fiction"? The answer is...it depends.
One of the stories for my upcoming collection, Horizons, for example, has a bit of an autobiographical twist to it. The story involves a down-and-out bookstore owner whose car crashes into a snowbank in the midst of a snowstorm. On a seemingly lonesome highway, allies suddenly appear when he least expects it.
At the risk of giving the story away, what happens next really did occur to me on a lonely highway in the middle of a snowstorm. I wasn't working at a bookstore at the time, but I was returning home from the late shift at a nearby restaurant. For the story itself, I altered the looks of the people that came to help, but I did not change how they worked that night to pull me out of my situation. You'll have to read the actual short story to see what happened that snowy night, but this made me think of an age-old question when reading other authors' works of fiction: how much of the story really happened to them or somebody they knew?
Sometimes, such as in the case of Hemingway, you can see where he got his inspiration for a book such as The Old Man and the Sea. Then if you try to read interviews with the author, the line between fiction and reality seems purposely blurred. It's pretty much impossible to tell what events in that book happened for real and which ones did not.
This tension sometimes leads to peculiar conversations. I remember being asked by a couple of people if the events in An Echo Through the Trees really happened to me. My answer: it depends how you look at it. Have I ever had to go and rescue someone who we thought was lost in the woods at night? Yes (turns out they weren't lost, though, but late in coming back to camp). The main character in that story also freezes his feet. Have I ever experienced that? Yes. I pretty much froze my feet after a long game of snow football in knee deep snow. Not fun. Have I ever written letters for months or even years and not gotten replies? Yes.
I could go on, but you get the idea.
Maybe a better way to describe this process is that an author will sometimes, but not always, draw from personal experience and then use those experiences as a sort of palette to paint with. The end result may not look anything like the original event, but in many cases, there might be a connection. Again, though, it really depends on the author as to how much "fact" they want to put into their fictional characters.