that is set for every user in the gnome preferences
for each user
from the gnome desktop
now if one of the ss's are running uncheck the checkbox
"Activate the screensaver after the computer is idle for..."
for the "black screen"
click on the "powerManager" button
under "On AC power"
select "never" for both
and if that dose not work ( in ScientificLinux 6.4 it dose not any more -- it did in 5.9 )
-- not installed by default
yum install gconf-editor
then run it as root
under the "Apps" on the left
and uncheck " idle_Activation_Enabled"
or set "Idle_Delay" to say 60 min or 360 min.