An early review of COD4 allegedly leaked in which OXM UK said the game had a 5 to 6 hour single player campaign when played at the normal difficulty. As we've seen already with games like Heavenly Sword, short playtime can get you a bad rep quick. Obviously COD4 will fall back on expansive multiplayer for added value, but my intent here is not to trash its game length, features, etc.
In the interest of developer-accessible census—what is the sweet spot for a single player campaign length? Personally, I like something that's about 8-12 hours...15 tops. Anything longer and I will absolutely never beat it. Anything shorter and if I feel gypped—not for what I paid as much as the accomplishment. Ultimately, gameplay always triumphs and exceptions apply. But for your average game, how long should it be?