I had zero interest at first, but I am lacking some stealth action in my gaming library (besides the Metal Gear Solid HD Collection). I am just Blacklist is going to be longer than Conviction, that's the only thing stopping me from picking it up day one.
The developers have been saying this is the biggest Splinter Cell to date, and that it's longer than Conviction's 6 hour campaign. I'm guessing around 9 to 12 hours based on the 13 single-player missions count, then factor in the comparable co-op/single-player content and the Spys Vs. Mercs adversarial multiplayer and you should be set for quite a while stealth-wise.
So basically from previews I can tell you that the whole game revolves around your HQ in a plane. Singleplayer, multiplayer and co-op are all acessed from it. The HQ can be upgraded through your play styles. There are main missions and side-missions? (see the achievement list). I'd say you've got at least 9 hours of singleplayer content +co-op and then there is multiplayer.
But I mean it's all relative. I can finish chaos theory in like 2 hours and the world record speedrun is 20 minutes and 23 seconds so....