Earlier today GamersNET visited The Creative Assembly at Gamescom for a presentation of Alien: Isolation. Gareth Ramsay from Square Enix, who hosted the presentation, told them more about how long the game will be.
He stated that The Creative Assembly has made a preview build that "contains about 10 to 15 percent of the game". It took the people who played this built three to six hours to complete it. Ramsay said most people were done in about 3,5 hours.
If this is true, it would be fair to estimate that the full game will be 25 hours long. That is of course impressive! The information is contained in a preview, which can be found by clicking the link.