According to the newest issue of PC Gamer (via CVG), Deus Ex: Human Revolution will last up to 25 hours.
The tidbit is apparently mentioned in the magazine by developer Eidos Montreal.
"So far with them playing about six hours every day, most of them don't complete the game," said producer David Anfossi.
"The ones that are more casual gamers, who are used to more 'corridor, cut-scene, corridor, cut-scene' type of gameplay - they play it that way for a while. Then suddenly, there's this new world that opens to them, and they start getting a bit more creative."
Deus Ex: Human Revolution releases this year for PS3, 360 and PC.