To change the elapsed time before your monitor automatically turns off

You must be logged on as an administrator or a member of either the Administrators or Power Users group in order to complete this procedure. If your computer is connected to a network, network policy settings might also prevent you from completing this procedure.
Open Power Options in Control Panel.
In Turn off monitor, click the arrow, and select the time you want.
Notes
To open Power Options, click Start, point to Settings, click Control Panel, and then double-click Power Options.
If Turn off monitor is not available, your monitor does not support this feature.
Using Power Options in Control Panel, you can adjust any power management option that your computer's unique hardware configuration supports. Because these options may vary widely from computer to computer, the options described may differ from what you see. Power Options automatically detects what is available on your computer and shows you only the options that you can control.