If you are using KDE:
Go to kcontrol, under the section Peripherals -> Display. Tab page Power Control. There you have three sliders and a check box.
Mark the check box labeled "Enable display power management", then choose your desires regarding the three timeouts. Press OK and you're done.
If you want to turn off your monitor without waiting, you can bind a keyboard shortcut to execute
xset dpms force off
Try it on a konsole. If it does not work then it usually means that your graphics card driver doesn't support it, or that it is not turned on in your Xorg.conf file.