Honestly it's impossible to really quantify this. Even with Halo 3 an individual's play time can vary dramatically based on difficulty level, modifiers like skulls, and whether or not it's played individually, cooperatively, or within the multiplayer portions of the game, which can amount to near infinite play time. While it's true that we view Halo 3: Recon as an extension to the Halo 3 game experience, it's too early to settle on a specific number of hours of gameplay.
First of all, while Halo 3: Recon did originate with the goal of building a 3 to 5 hour campaign experience, I can safely say that as is always the case with Bungie, the team is continuing to put more and more into it and the final product will end up being some order of magnitude more than the initial design. Game Informer also touches on the new structure to the campaign experience and how there are basically two separate components to the campaign. The initial play time targets were focused on one of those components. And lastly, there are still plenty of things we're not talking about with Recon that will further enhance and add to the experience, play time, and replayability of the final package.