Since "intelligence" is ill-defined and "consciousness" can't be measured, some disambiguate with the term "technological". Take what I said to mean that if you prefer. :)
My point: If technological civilizations were common, we'd probably expect to have seen a few pop up in Earth's long history of life. Since it's only us, it's not likely to be more common than "one for every 3.5 billion years of life", and there's no bound how much rarer it could be than that.
