the vizqlserver.session.expiry.timeout should work for you.
tabadmin set vizqlserver.session.expiry.timeout 10
with this being said, it's not an exact timeout. Rather Tableau has internal cleanup tasks that run at set intervals and if the timeout is greater than the value then it's cleaned up.
and you should be able to see the custom settings within your install directory (i.e. g:\tableau\tableau server\config\tabsvc.yml)