System : Ubuntu MATE 15.04
Machine : Toshiba Radius P55W-C5200
External monitor : Lenovo 23" IPS LED HD
uname -a
3.19.0-25-generic #26-Ubuntu SMP Fri Jul 24 21:16:27 UTC 2015 i686 i686 i686 GNU/Linux
The option in Power management:
"Put display to sleep when inactive for" x minutes
does not turn off the monitor
[EDIT]
This seems to be a MATE only issue, as explained here:
External monitor not going to sleep
The behaviour (monitor not sleeping) only happens when the laptop lid is closed. Keeping the laptop lid open the external monitor does go to sleep.