SELECT JOB_NAME,
       avg(extract(DAY FROM run_duration) * 24 * 60 * 60
               + extract(HOUR FROM run_duration) * 60 * 60
               + extract(MINUTE FROM run_duration) * 60
               + extract(SECOND FROM run_duration))
FROM DBA_SCHEDULER_JOB_RUN_DETAILS
WHERE JOB_NAME = 'my_job_name'
GROUP BY JOB_NAME;