As far as I understand a PFS scenario usually takes around 4 hours to be completed but I wonder how much time (average) in hours an AP module (adventure) usually take?
It varies a lot.
I am running Curse of the Crimson Throne and my group, playing once a week for about 4 hours, took 3 months just for book 1.
But playing Second Darkness we are already nearly 2 years and are still just midway through I think.
It depends on your group's style of play.
When I'm the DM, it takes about 5-6 sessions of about 6 hours to finish a chapter. When played once a week (with a few exceptions, so let's say 4 sessions in 5 weeks), it takes me about 8-9 months to finish an AP.
Im running Skull and Shackles and book 1 took 12 weeks of 3hour sessions, we are on book 2 and not even half way through and it has been 13 sessions of 3 hours so far.
A lot of it depends on whether you follow the book and don't deviate or you like to add things in of you're own or let the players do what they want (part 2 of s&s is a sandbox just waiting to happen, pretty much the same for part 3 of serpents skull if I remember too so if you embelish these parts it can take a lot longer)
I would say though maybe a minumim of say 30 hours for each book so somewhere around 180hours overall maybe but thats a very vague guideline.
AP's have great potential if you play the right one but they do take a heck of a lot of work by the GM to run but the rewards are worth it I think.
I've beem running Rise of the Runelords since May 2011, and we're only about halfway through. We play once a week for a scheduled 4-hour session, but we rarely get more than 3 hours of actual gaming in. We usually end up cancelling one session a month due to scheduling conflicts-- and we coudn't get a quorum to play over the entire month of July 2012.
I'm expecting it to take three years of real-world time to complete, but your miles may vary!
My groups seem to go at a rate of ~1 book a month pretty consistently. I don't add anything to them, and we tend not to do super heavy roleplaying. RPing happens, it's just fairly simple and light. Even with heavier RP, I'd guess at no more than ~1.5 months for us per book.