At the moment it uses just a few seconds of inactivity after that ubuntu reduces the brightness of my screen. Where do i find the option to increase this value?
This is the "Regard computer as idle after" slider on System → Preferences → Screensaver:
Or the gconf key
/apps/gnome-power-manager/backlight/idle_dim_time, which you can set thusly:
gconftool --type int --set /apps/gnome-power-manager/backlight/idle_dim_time 120
which will set the timeout to 120 seconds.
You can change this setting in the menu "System / Settings / Power management". This setting can be changed if you're running on battery.
(I have the Swedish GUI so I hope I remembered the English menu names correctly! ;)
It looks like you must change the value of this key (which I made it a mandatory key, because I didnt want the users to change it.)
here's the key in gnome2 when your run the gonf-editor : /desktop/gnome/session/idle_delay