IN 12.04 at least, this seems to be tied to Xorg’s blanking which by default happens after 10 minutes/600 seconds. It can be unset via xset on the commandline, but I finally managed to do this via xorg.conf, like so:

jani@saegusa:~$ cat /etc/X11/xorg.conf.d/*
Section ”Extensions”
Option ”DPMS” ”Disable”
Option ”MIT-SCREEN-SAVER” ”Disable”
EndSection

Section ”Device”
Identifier ”Intel Graphics”
Driver ”intel”
Option ”DPMS” ”Disable”
EndSection

Section ”ServerFlags”
Option ”NoPM” ”true”
EndSection

Section ”ServerLayout”
Identifier ”X.org Configured”
Option ”BlankTime” ”0”
Option ”StandbyTime” ”0”
Option ”SuspendTime” ”0”
Option ”OffTime” ”0”
EndSection

It could be that just the ServerLayout section is actually required (and I’m not sure if other graphics drivers even have a DPMS option of their own like intel does), as none of the other sections by themselves nor together prevented the screen from turning off. (The most confusing one is MIT-SCREEN-SAVER, which I thought was exactly what the xset timeouts were controlling, but apparently not so, as disabling it does not prevent the issue either.)