Increase screen idle dim timeout

This command should set the time to 60 seconds:

gsettings set org.gnome.settings-daemon.plugins.power idle-dim-time 60

In earlier Ubuntu versions, this setting can be found with the gconf-editor under gnome-power-manager. See also this thread.

The answer marked above from 'lumbric' no longer works for Ubuntu 14.04.

If you need to do this from a command (e.g. for a script wrapping something that plays video), you now need to run:

gsettings set org.gnome.desktop.session idle-delay <seconds>

also useful in such a case might be disabling the screen lock:

gsettings set org.gnome.desktop.screensaver lock-enabled false

In 12.04, the setting using GUI now exists via System Settings → Brightness and Lock.