With a recent update (can't pinpoint which because I have the nasty habit of updating often but only rebooting once in a blue moon) my Lenovo X201 monitor started dimming the brightness after something that felt like 30s without input from mouse or keyboard.
This smelled like GNOME gsettings and after mucking around I found how to deal with the issue:
$ gsettings set org.gnome.settings-daemon.plugins.power idle-brightness 300
Basically gsettings can used to set the key (I've choosen to change from the previous set 30 to 300 seconds) of a target schema (idle-brightness) within the org.gnome.setting-daemon.plugins.power schema directory.
For more on gsettings:
$ man gsettings
$ gsettings help