CoD4's Singleplayer is 5-6 hours...

[Entroper]

Dallinor said:
For an FPS 5-6 hours isn't too bad. I can complete Half Life in about that time frame, which goes to prove that quality is better then quantity (as long as the game isn't too short).

The multiplayer will also add a huge amount of replay value.

Half-Life in 5 hours?  I really doubt that.