Here you go try this
HKEY_CURRENT_USER/Controlpanel/Backlight/CMCS_SLCDTimeout
It is in milliseconds (60000 for 60 seconds). You will have to reboot the phone after changing it for it to take effect. It you really don't want it to ever go off, set the value to 0, then toggle back once more and it will then go to the largest number available (I think it works out to like a month).
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