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1. Choose Apple menu > System Preferences.
2. Click Security & Privacy, and then click General.
3. Click the lock icon at the bottom left to unlock it, and then type an administrator name and password.
4. Click Advanced.
5. Check “Log out after ... minutes of inactivity.” 
6. Set the amount of inactivity time before the system automatically logs out. 

   1. Choose a time that's not long enough for an unauthorized person to use your computer, but not so short that if you stop working at your computer for a moment, the screen saver locks it.

7. Click OK.
8. Click the lock icon again to prevent further changes.