It took me, on the highest initially unlocked difficulties:
GoW = 8 hours (whole game was too easy but the Doppelganger battle was too hard)
GoW II = 14 hours (difficulty still a bit too easy, no horrendous difficulty spikes though)
GoW III = 10 hours (difficulty was pretty much perfect)
Chains of Olympus = 5 hours (too easy, no noticeable difficulty spikes)
Ghost of Sparta = 8 hours (a couple difficulty spikes exasperated by the series' worst camera problems)
Tried it on Titan?
OT: The place with lots of enemies is really easy. There are two types of enemies and one of them can be thrown. Grab one of them and do the square throw. Repeat it until there aren't more enemies.
As a rule of thumb in GoW, if an enemy can be grabbed, go for it and use the square or triangle throw, those are usually the best ones. All enemies in a wall or ceiling can be thrown (even the hands at Hades). And never jump with gorgons around.
Edit - Kratos becomes completely invincible during the animation of Poseidon's Rage (GoW 1&2) and Atlas Quake (GoW 2).