[quote name='Npat12']How long is the gameplay on avg.?[/QUOTE]
Depends. If you just play it once to finish the story, probably 8 hours. But there's so much to do, and the world is a lot of fun to explore, so you'll probably end up playing through it more than once. I played through it three times to get everything, and that obviously extended the playing time. There's also an online mode called Terror is Reality, which is a gameshow where you compete against other people to rack up the most kills in special environments. I don't care for it, but it was fun seeing the different game modes, so that might occupy you for another couple of hours.
The game is imperfect and has some bugs, and the control isn't great. But overall, it's one of my favorite games due to its great sense of humor, zombies, and huge world.