Joomla will automatically log the user out after the period you've set in the "Session Timeout" period in the Global Configuration setting. If the user needs to be logged in to do something, after this period lapses without any activity, they'll need to log in again.
We'd recommend not setting that value too low (forcing frequent logins) as the post to wall on login can get spammy if it's done too frequently. Just a warning, but it's your site and options to come up with the best combination of promotional efforts that work for you.
Hope this helps, and good luck!
Login does not work on www. site
