Log individual users out of a period of inactivity?
The MacEnterprise list has discussed this topic recently, Forcing Log-Off For Idle Users.

autologout Defaults

John NcNair suggested the following autologout defaults:

defaults write /Library/Preferences/.GlobalPreferences com.apple.autologout.AutoLogOutDelay -int 900 
defaults write /Library/Preferences/.GlobalPreferences com.apple.securitypref.logoutvalue -int 900 


Limitations

Bruce Carter mentioned...


  Note that nothing using Apple's internal mechanism for this will work if 
  the user has unsaved files open.  It will simply stop at the dialog box and 
  time out.


See the related question How can I automatically log out the guest account after a period of inactivity? for other approaches and tools.