I use a short shell function for this:

  take-a-break ()
  {
    if [ $# -lt 1 ]; then
        length="60"
    else
        length="$1"
    fi

    echo "Take a break in $length minutes at $(date --date="+$length minutes" +%T)."

    sleep "${length}m"
    gnome-screensaver-command -l

}

Replace gnome-screensaver-command with whatever screensaver you use.