There is a comment from the developers indicating that an average playthrough on normal difficulty should take between 15-20 hours. A playthrough on classic difficulty takes "a lot longer."
"How long will a normal playthrough of XCOM: Enemy Unknown take?
Garth: Anywhere from 15-20 hours on normal, and a lot longer on the higher difficulties. From day one, we did build this to be completely replayable. In this regard, it’s kind of long for a replayable game. We totally expect people to replay.
Jake: The difficulty level changes. If you play on normal, it will take 15-20. If you bump up to Classic (a higher difficulty) not only is it longer, but it’s going to be a completely different experience because you’ll need to focus on different elements to overcome the same obstacles."