What is the average duration time of the films starring PENELOPE GUINESS?
average duration time = AVG(length)
SELECT AVG(T3.length) FROM actor AS T1 INNER JOIN film_actor AS T2 ON T1.actor_id = T2.actor_id INNER JOIN film AS T3 ON T2.film_id = T3.film_id WHERE T1.first_name = 'PENELOPE' AND T1.last_name = 'GUINESS'