Find the following file:
Web.config (located in <install directory>/Tenants/00000000-0000-0000-0000-000000000000)
<add key="autoLogoutTime" value="30" />
Time in minutes after which inactive users are automatically logged out. Supported values range from 10 to 540.
Note: Editing of config files is officially unsupported. Config files will likely reset to their defaults after upgrades. Post questions, feedback and comments here.