According to Security in the Cloud
Tableau Online enforces a session timeout of two hours. After a short period of inactivity, users are signed out. Users will need to re-authenticate after not using Tableau Online for a period of 12 hours. These are Tableau Online system settings that cannot be modified.
By the way, I've moved this post to Server Administration , which might be a better place for your question.
Thanks Lei. I look forward to possible solutions instead of just having the default settings.