Note, the following procedure no longer works in Ubuntu 14.04 and newer.
Ubuntu 12.04 screen dim timer (NOT Ubuntu 14.04/16.04)
The idle screen timer in Ubuntu comes set to dim the laptop screen automatically. You can check your current idle timer by typing in Terminal:
gsettings get org.gnome.settings-daemon.plugins.power idle-dim-time
To SET the idle screen dim timer in Ubuntu 12.04 to a longer time (e.g. 90 seconds) type
gsettings set org.gnome.settings-daemon.plugins.power idle-dim-time 90