To get the information about the average number of days a user keeps a particular book, you need to perform the following steps:

1. Add the average_book_occupation() method to library.book:

def average_book_occupation(self):
    ...

2. In the method, write the following SQL query:

sql_query = """
    SELECT
        lb.name,
        avg((EXTRACT(epoch from age(return_date, rent_date)) / 86400))::int
    FROM
        library_book_rent AS lbr
    JOIN
        library_book as lb ON lb.id = lbr.book_id
    WHERE lbr.state = 'returned'
    GROUP BY lb.name;"""

3. Execute the query:

self.env.cr.execute(sql_query)

4. Fetch the result and log it:

result = self.env.cr.fetchall()
logger.info("Average book occupation: %s", result)

5. Add a button in the form view of the library.book mode to trigger our method:

<button name="average_book_occupation" string="Log Average Occ." type="object" />

Don't forgot to import logging in this file. Then, restart and update the my_library module.