What is the average year for a user to be upgraded to elite user?
AVG(user_yelping_since_year) where user_id from Elite;
SELECT CAST(SUM(T2.year_id - T1.user_yelping_since_year) AS REAL) / COUNT(T1.user_id) FROM Users AS T1 INNER JOIN Elite AS T2 ON T1.user_id = T2.user_id