(How) can I change the time until the screensaver kicks in from the command line?

reg add "HKEY_CURRENT_USER\Control Panel\Desktop" /v ScreenSaveTimeOut /t REG_SZ /d 600 /f

The value 600 is in seconds, = 10minutes Interestingly, as pointed, this works only the first time. On next change, you need to make a new key like so :

reg add "HKEY_CURRENT_USER\Control Panel\Desktop" /v ScreenSaveTimeOutTemp /t REG_SZ /d 300 /f

Then delete the old one

reg delete "HKEY_CURRENT_USER\Control Panel\Desktop" /v ScreenSaveTimeOut /t REG_SZ /d 600 /f

Then copy the new one to the right name

reg copy "HKEY_CURRENT_USER\Control Panel\Desktop" /v ScreenSaveTimeOutTemp "HKEY_CURRENT_USER\Control Panel\Desktop" /v ScreenSaveTimeOut

Hopy this works, and you do not destory your computer, but since you did not have this registry value, i don't think deleting it can be that horrible.

REG documentation here: http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/reg.mspx?mfr=true