It should be completely dark by 8:15ish/8:30ish. I always remember waiting for Fantasmic or Illuminations to start and thinking "This can't be dark enough for this show to work" and then within 15 before the show starts, it's completely dark.
I live in Georgia and in June it is not completely dark until 9 PM. It will be the same for Orlando. The sun sets around 8:30 - but it will still be "twilight" until right at 9.