Enter a short time in the box under "Screen Saver Settings" for
"Start after ___ Minutes" - for example, I use 4 minutes.
Now make sure the option is checked to "Use power management"
and enter a short time in the box "Shutdown monitor ___ minutes after screen
saver has started. I use 2 minutes.
You can also open the advanced
screensaver control panel by typing xscreensaver-command -prefs
at a command prompt.