Use the "Time until screen saver starts" slider to set an appropriate delay.
Step 2 of 2: Set the Energy Saver Settings:
Click "Show All."
Click the "Energy Saver" icon.
If you want to put your entire computer to sleep, choose the time using the "Put system to sleep when it is inactive for:" slider. Note: when your computer is asleep, it uses very little power, and it is essentially turned off (i.e. it can't do anything).
If you only want the screen/monitor to go to sleep, click the box labelled "Separate timing for display sleep" and choose an appropriate time. Note: if you choose a time that is shorter than the amount of time before the screensaver starts, you will never see the screensaver!
For extra savings, also click the button labelled "Separate timing for hard disk sleep."