How do I change the time my PC can stay on unattended before it switches off or goes into sleep mode?

Two things to change are the screen saver and your power options.  For the screen saver right-click an empty spot on the desktop and click Personalize.  Then click Screen saver.  There you can set how long the computer will wait before the screen saver comes in.  From there you can click Change power settings and tweak numerous settings that are there.

 I hope this helps.  Good luck.

Change Power Settings

How to:

• Set your computer to go to sleep after a specified period of inactivity. For more information, see http://windows.microsoft.com/en-US/windows7/Turn-off-a-computer-frequently-asked-questions
• Adjust the brightness of your display.
• Choose when to turn off the display during periods of inactivity.

1. Open Power Options by clicking the Start button, clicking Control Panel, clicking System and Security, and then clicking Power Options.
2. Under the plan that you want to change, click Change plan settings.
3. On the Change settings for the plan page, choose the display and sleep settings that you want to use when your computer is running on battery (if applicable) and when it's plugged in.
4. For more options, click “Change advanced power settings” and for even more click "Change settings that are currently unavailable" (if available).
5. When you’re done click OK.