Well all the comments from Tam I've heard was 12-15hours. It was Lair that was said to be 10-12 hours. SO i don't believe 6hours in the least.
I did a play though on the day (and before) we sent the version which became the US master to format.
It took me 9.5 hours on normal only getting about half the glyphs and I can (almost) literally do the army portions of the game with my eyes shut (and have been working on it for 3.5 years so might have picked up a few hints to the rest of the game as well ;-) )
So if your my level of expertise (i'd admit to not be being the best gamer) I'd be surprised if you can do it under 10 hours.
Hell mode would take considerable longer, I'm not sure I could do it.