Jenny,
Currently in v11 there isn't an automatic timeout for users. The best way is when they are done with their session to do use the logoff at the top right hand side in order to log out. (The bad)
(The good). In v12 with Frontdoor being enabled (replaces the currently user security in v11) there is a timeout period that a user will hit that will automatically log them out of the system.
Thanks,
Ryan