One Out of Two Portal 2 Owners Haven't Beaten it.

"Little more than half the players who bought the game on Steam have finished it and more than one in ten of them haven't even gotten to that very first puzzle in the solo campaign, according to the service's achievement-tracking stats. Only 55.7% of players had earned the final achievement of the solo game. Only 89.0% had even earned the first achievement, which can be acquired with virtually no effort after the game starts." - Kotaku

It's more understanding that 55.7% players haven't earned the final achievement of the solo campaign, but that 11% haven't earned the first achievement of the game? That basically means that you bought the game and well, that's it. Maybe it's from Steam pre-orders, who knows. The solo campaign itself is between 6-10 hours to beat. Which for a gamer could be a matter of one sitting or just a few. To read up a bit more on Kotaku's article on the statistics and their play through, go here!