When did the creator of the list "250 Favourite Films" last updated a movie list?
250 Favourite Films refers to list_title; last update refers to list_update_date_utc;
SELECT T2.list_update_date_utc FROM lists AS T1 INNER JOIN lists_users AS T2 ON T1.list_id = T2.list_id AND T1.user_id = T2.user_id WHERE T1.list_title = '250 Favourite Films' ORDER BY T2.list_update_date_utc DESC LIMIT 1