I can't change the time for display dim

Hi all, I’m very impress with the latest 11.1 release, it works as a treat on my acer 5920 laptop; the only thing I have not manage to do is to change the power settings for dimming the display for longer than the default 5 minutes, which is very annoying when you are watching a movie. I tried to change it using the KDE4 settings manager, both as sudo and as normal user, but no luck. Any body has any suggestions? Thanks in advance.