Ok ill start of by saying that I got all the achievement on normal legit. Apart from hard - before any one complains.
Right before starting my hard play through I looked on the internet to see how long it would take and couldn't find anything so in case any one was wondering (like myself), it took me around 7hours30
Im neither good nor bad at this game so im sure it could be done a lot quicker.
Check this thread out if you want to go through it with cheats and get achievements - http://www.xbox360achievements.org/f...d.php?t=155872
(hats off to aCzarX for the thread) and i can confirm that you will get the achievement still if you follow the thread above.