How can I enable screenlock without turning off the monitor? (Ubuntu Gnome 14.04)
This is not possible anymore in Gnome 3. You would have to execute gsettings set org.gnome.settings-daemon.plugins.power idle-dim-time [seconds]
(possibly as lightdm as this is the user who runs the lockscreen) however this key doesn't exist anymore.
A possible workaround Khurshid mentioned is to install XScreenSaver (and set the timeout from there):
How can I change or install screensavers?