The iTunes ones are 25-30 minutes each. They're supposedly verbatim from the comic, but not exactly - because sometimes the narration/dialogue when read is actually too long for the animation (which is as faithful as it can be to the comic), some less-than-necessary pieces of dialogue is cut.
For example, Rorschach's opening narration:
--ORIGINAL COMIC VERSION--
"Rorschach's Journal. October 12th, 1985: Dog carcass in alley this morning, tire tread on burst stomach. This city is afraid of me. I have seen its true face.
The streets are extended gutters and the gutters are full of blood and when the drains finally scab over, all the vermin will drown.
The accumulated filth of all their sex and murder will foam up about their waists and all the whores and politicians will look up and shout 'Save us!' ... and I'll look down, and whisper 'No.'
They had a choice, all of them. They could have followed in the footsteps of good men like my father, or President Truman. Decent men, who believed in a day's work for a day's pay.
Instead they followed the droppings of lechers and communists and didn't realize that the trail led over a precipice until it was too late. Don't tell me they didn't have a choice.
Now the whole world stands on the brink, staring down into bloody hell, all those liberals and intellectuals and smooth-talkers ... and all of a sudden, nobody can think of anything to say."
--MOTION COMICS VERSION--
"Rorschach's Journal. October 12th, 1985: Dog carcass in alley this morning, tire tread on burst stomach. This city is afraid of me. I have seen its true face.
The streets are extended gutters and the gutters are full of blood and when the drains finally scab over, all the vermin will drown.
The accumulated filth of all their sex and murder will foam up about their waists and all the whores and politicians will look up and shout 'Save us!' ... and I'll look down, and whisper 'No.'"
The reason for this is that actually reading all that takes too long - "I'll look down and whisper 'No'" comes when the frame is finally above the cop looking out the window saying "Hmm" in the motion comic, while it hasn't even gotten higher than the guy with the hose and the "End is Nigh" man at that point in the graphic novel.
But for people who want the movie to be verbatim from the comic - well, if you add 25 minutes together 12 times (for all 12 chapters), it'd be 5 hours long. And even then, some things are still cut for time. A truly word-for-word adaptation would probably be 5 hrs 30 mins or so. A bit long for a movie.
The motion comic is very good, though, for anyone who wants to read it before you see the movie but aren't a fan of reading comics (like one of my roommates), or if you want to re-read in a more streamlined way. It's like listening to a slightly-abridged audio book, with pictures included and animated. If only they had a different person do each voice (watching Laurie talk with a man's falsetto is a bit odd), and animated the mouths, it'd be pretty much perfect.