This actually depends on genre:
Epic/heroic fantasy: 100,000-120,000
All other genres: 80,000-100,000
These aren't so much averages as the lengths that will give you the best chance of publication as a new author. Experienced writers can often get away with longer or shorter.
Oh and never, ever *ever* say 'fiction novel' around editors or agents. If you include that in your query they won't even read it. Trust me. If it's a novel, it's fiction.