MS created a KB article about this issue:
Monitor powers off after 1 minute when PC is locked
This behavior is by design in Windows 8. By default, when the console is locked, Windows waits for 60 seconds of inactivity before powering off the display. This setting is not configurable using the Windows user interface.
Using the PowerCfg.exe utility, you can configure the display timeout
used when the PC is in an unlocked state as well as when it is at a
locked screen. From an administrative command prompt, the following
commands can be used to control the display timeout:
powercfg.exe /setacvalueindex SCHEME_CURRENT SUB_VIDEO VIDEOIDLE
powercfg.exe /setacvalueindex SCHEME_CURRENT SUB_VIDEO VIDEOCONLOCK
powercfg.exe /setactive SCHEME_CURRENT
The VIDEOIDLE timeout is used when the PC is unlocked and the
VIDEOCONLOCK timeout is used when the PC is at a locked screen.