How long does it take to play Arkham Horror?
Well I can tell you my personal experience - I bought it because I knew lots of friends and acquaintances in my past were fairly obsessive about it, and organized an 8-player game one evening with a bunch of beginners.  Four or five hours later, we were maybe halfway through the game?  One or two of the group had gotten pretty into their characters and quests, but some of the others had completely given up, it was just too much.  Admittedly this was a very "casual" set of players, not dedicated boardgamers, but it's a cautionary tale.

I'd say you wouldn't want to break this out except amongst people who are pretty serious about boardgames, and preferably people who have done some roleplaying - Call of Cthulhu would be ideal, obviously, but in general terms I really would class this as a roleplayer's boardgame.  It definitely seems advisable to learn the rules with a smaller group than 8!  I expect that games will speed up as everyone becomes familiar with the mechanics and strategies, but I'd still expect to be talking 3+ hours.  

I don't see the point of breaking it up into multiple sessions really - part of the fun of the Lovecraftian universe is slogging onwards to the end of your quest despite encroaching exhaustion and madness!  If you're looking for a light, fun game that you can whip out at short notice and then play for a couple of rounds before putting away again, Arkham Horror would seem to me like a really odd choice!