Let's assume each billion of years in the universe there is an equal chance of intelligent species developing
(Of course that's not completely true, but it doesn't break the point)
Say, what are the chances that we'd develop during the first 1% of the age of the universe? 1%
So it is 50% likely we have develop during the second half of the age of the universe. If we had to take a shot, the mean value, 50% is the most reasonable to assume
The universe is 13 billion years long. So we can assume 13 billion years is what it has left. Say goodbye to your family and loved ones, the end is coming