I believe the dark-window generally refers to the brief outage that can occur as the packet-forwarding engines restart due to new microcode being pushed down and installed on them. It seems to vary in length based on the platform, the JunOS version being upgraded from/to, etc. I suggest if you need absolute measurements, you get your hands on some IXIA or Spirent test-gear and the platform in-question. Alternately, your Juniper sales-engineers might be able to get you some answers on your specific platform.
Ron