This is probably the best solution. No need for screensaver tweaking and running.

Install sudo apt-get install xprintidle

Put this script into autostart:

#!/bin/bash



idletime=$((1000*60*60*2)) # 2 hours in milliseconds



while true; do

    idle=`xprintidle`

    echo $idle

    if (( $idle > $idletime )); then

        #sudo shutdown -P now

        dbus-send --system --print-reply --dest=org.freedesktop.login1 /org/freedesktop/login1 "org.freedesktop.login1.Manager.PowerOff" boolean:true

    fi

    sleep 1

done

The comments and @Jobin 's answer did lead me to investigate myself again and I actually found a unfinished script with xprinttime but without any loop in the script. Probably not finished this because my linux/bash knowlege was not good at the time. I also did put the script I had in rc.local or something like that triggered the shutdown on boot. Thanks to @Jobin for the reminder how to add startup apps in XFCE, I already knew this but ... and credits for the dbus thing, never saw that, better then shutdown since it not requires root.