In an interview with OXM, Splinter Cell: Conviction creative director Maxime Beland reveals that the game should take around 12 hours to complete on normal difficulty. The game will also ship with a "realistic" difficulty that will be soul-crushing, from the way Beland describes the mode:
"...Realistic is going to be really f*cking hard - you're going to need stealth, two bullets is going to kill you, the enemies are going to be super-lethal."
There seems to be a big difference between the two modes. Normal is so everyone can finish the game. That's fine. I'd appreciate a hard mode between that and realistic, however. Something not as brutal, but more challenging than normal. What do you think?