By default in Windows 8 and 8.1 after you lock your screen, the display will timeout and power off after 1 minute of no activity, requiring you to wake the screen to login again. [more]There is no way to modify this through the GUI by default. You can, however, add a registry key to enable control of this setting. The reference site below has .reg files you can download and use to make the registry change or you can make the change manually if you are familiar with modifying the registry. If going the manual route, set the DWORD value Attributes to 2 (a value of 1 hides the entry). This value controls the visibility of the option ‘Console lock display off timeout’ under Advanced Power Settings.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Power\PowerSettings\7516b95f-f776-4464-8c53-06167f40cc99\8EC4B3A5-6868-48c2-BE75-4F3044BE88A7\Attributes
To modify the timeout setting, go to Power Options, choose ‘Change Plan Settings’, and then choose ‘Change advanced power settings’. Under Display, you will now see the option ‘Console lock display off timeout’.
Reference: http://www.eightforums.com/tutorials/8267-power-options-add-console-lock-display-off-timeout.html