Ubuntu automatically makes your display dim when you are idle for 30 seconds. Yes this saves battery life but for me 30 seconds is really annoying. And the good thing is this default value can be easily changed from gconf-editor.
- Press ALT+F2
- Enter gconf-editor
- In gconf-editor browse to apps > gnome-power-manager > backlight > idle_dim_time
- Hit enter
- Change the integer value in seconds.