Note, the following procedure no longer works in Ubuntu 14.04 and newer.
Ubuntu 12.04 screen dim timer (NOT newer Ubuntu versions)
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