Maybe have look at ...
XML Export by Kage
In general you can add %_length_seconds% per file and divide the sum by %_total_files%.
Try this export file (one line):
$filename($getEnv('USERPROFILE')'\Desktop\Export.Time.Files.Avg.txt',UTF-8)'Total Time [s]: '%_total_time_raw%', Total files: '%_total_files%', Average Duration [s]: '$div(%_total_time_raw%,%_total_files%)
Output examples:
Total Time [s]: 28, Total files: 4, Average Duration [s]: 7
Total Time [s]: 344, Total files: 4, Average Duration [s]: 86
Total Time [s]: 330, Total files: 2, Average Duration [s]: 165
Total Time [s]: 507, Total files: 3, Average Duration [s]: 169
Total Time [s]: 146, Total files: 29, Average Duration [s]: 5
What do these values tell us?
DD.20170718.1120.CEST, DD.20170719.1846.CEST