Which one is correct, or are both of them fine?
- "It would take the average person 10 days to read this novel"
- "It would take an average person 10 days to read this novel"
"An average person" is a phrase with an article, adjective, and noun acting normally with each other. It means "any person who can be considered average."
"The average person," as the article "the" implies, is referring to a specific idea that is shared among all people (assuming that's possible) of what an average person must be like. In other words, it's referring to an archetype of the "default" person—the type of person any of us could be if we didn't have any eccentricities or distinguishing characteristics. "The average person" is a common phrase that, taken as a unit, has the described understood meaning.
All that said, the ideas expressed by both sentences are virtually identical.
the when you mean something specific. Use
a/an when you mean something in general.
In your case, both sentences make sense. I think the difference is that the first example focuses on specific average person, while the second example on a person in general. You may need to refer back to the original context before telling which is better.
Also please take some time to check this page