like i said, i think the majority of newer phones with colour screens time out to preserve the screen
i expect they will allow you to keep the light on longer with future phones, but right now for the 6680 it just doesnt happen.
also, there are other phones that light up the screen for you. i notice you have a ck-10 from another post, this wont partly because it does not seem to support the 6680 and partly because it does not have any external controls, you would need to touch the phone to light it up.
kits like the 616 and ck-7w look like they have a remote control, so using that would trigger the backlight for you when you need to find contacts etc