For the user who gave the most number of long reviews, what is his/her averge ratings of all review?
long reviews refers to review_length = 'long'; most number of long reviews refers to MAX(COUNT(review_length = 'long')); average ratings = AVG(review_stars);
SELECT CAST(SUM(T1.review_stars) AS REAL) / COUNT(T1.review_stars) FROM Reviews AS T1 INNER JOIN Users AS T2 ON T1.user_id = T2.user_id WHERE T1.review_length LIKE 'Long' GROUP BY T1.user_id ORDER BY COUNT(T1.review_length) DESC LIMIT 1