Comment 2 for bug 1055874

@Sparhawk
what's the result of
#gsettings get org.gnome.settings-daemon.plugins.power idle-dim-time
?

idle-dim-time will affect the time you set on those settings, you need to plus the dim time.

if idle-dim-time is 30, and sleep-display-ac is 60
your monitor will turn off after 90 secs inactive.

and whenever you set sleep-display-ac by gsettings , you also need to set

[org.gnome.settings-daemon.plugins.power]
sleep-display-ac
sleep-display-battery
and

[org.gnome.desktop.session]
idle-delay

these 3 values need to be same in the gsettings.