Nobody can tell you without pulling numbers out of thin air. That's the point. The only thing that can be said is that the probability of intelligent life arising is greater than 0, which follows from our own existence. Beyond that, any claims about the likelihood of intelligent life are just 20th and 21st century versions of "there must be a God because the universe couldn't have arisen by chance."
Strictly speaking our own existence doesn't even tell us the probability is greater than zero. For an event to have probability zero doesn't imply it can't occur. If a number is chosen from a uniform distribution on the reals between zero and one, whatever the result is the probability of that exact result occurring was zero.
