Ok so it seems that the key in org.gnome.settings-daemon.plugins.power idle-dim-time is gone in the new Ubuntu version. Since the old method Increase screen idle dim timeout doesn't work anymore are there any other ways to change this?
It looks like the setting for idle dim time has changed schemas. Use org.gnome.desktop.session idle-delay:
gsettings set org.gnome.desktop.session idle-delay <value>
For the future, a great way to find a setting is to pipe list-schemas or list-recursively to grep. That's how I figured this one out:
gsettings list-recursively | grep idle
Hope that helped!
Apparently Gnome developers decided that user does not need any way to customize idle dim time and removed this option in this commit. After this patch,
dim-idle-time was computed based on
idle-delay (see @ruby answer) by dividing it by three. Few days later they decided that dividing by 3 is not good and changed this factor to 4/5 in this commit. Even though those arbitrary divider proved to be, well, arbitrary, they didn't restore any possibility to change this by user.
Note that Ubuntu is now using unity-settings-daemon which is a fork of
gnome-settings-daemon with exactly the same code regarding screen dimming.