Comment 9 for bug 46575

you can easier do it by dropping the line:
Option "DPMS"
from your xorg.conf file
to fix it right is actually quite tricky since xorg overrides the screensaver here, so even though setting a proper value for it wont help unless we remove that default line ...
but ... removing it will then cause powermanagement to fail on login manager screens or on window managers that have no power manager at all.