How can I trigger the screen saver manually from the command line? I'm using GNOME 3.8.4 on Fedora 19, but the solution should not necessarily be specific to this distro.
The details on how to do this were found here in this blog post titled: Locking the screen from the command line in Gnome 3.8.
dbus-send command can be used to send this message, in this case we're sending the "Lock" message to the screensaver.
$ dbus-send --type=method_call --dest=org.gnome.ScreenSaver \ /org/gnome/ScreenSaver org.gnome.ScreenSaver.Lock
Typically this same message will be sent when you've configured for this particular timeout to occur through the desktop settings.
You can check the amount of idle time required before the locking will automatically get triggered, from the GNOME Control Center, Settings -> Power -> Blank Screen.
You can check the value of this delay from the command line like so:
$ gsettings get org.gnome.desktop.session idle-delay uint32 600
Also you can change it via the command line, or through the GNOME Control Center.
$ gsettings set org.gnome.desktop.session idle-delay 300