OS X Mountain Lion: Set your Mac to log out when not in use
Keep your Mac password protected and safe from prying eyes when your computer is idle. You can easily set up your Mac OS X computer to automatically log-off after a predetermined period of time.
In the Mac OS X Finder or Desktop, select the Apple Menu in the upper left-hand corner of your screen. Select “System Preferences” and release your mouse/trackpad to launch the “System Preferences” application.
Single click the “Security” icon in “System Preferences” to launch the “Security” preferences.
Click the “Log Out After” checkbox in the “General” tab of the “Security” preferences.
Input, using your number keys or the time toggle bar, the amount of time, in minutes, before your Mac will automatically log out after inactivity. Close the System Preferences.
Read more: How to Set a Mac to Log out Automatically | eHow.com http://www.ehow.com/how_4577940_set-mac-log-out-automatically.html#ixzz2IICwRkMx