Hmm...
Iron Man (2008) - 126 minutes
The Incredible Hulk (2008) - 112 minutes
Iron Man 2 (2010) - 125 minutes
Thor (2011) - 114 minutes
Captain America: The First Avenger (2011) - 124 minutes
The Avengers (2012) - 143 minutes
Phase I Total: 12 Hours and 24 minutes
At this point, you would need a completely free day and a comprehensive food solution, and a policy on bathroom breaks if you're not doing it by yourself.
[Assuming similar runtimes to previous films or similar properties by the same director]
Iron Man 3 (2013) - 123 minutes
Thor 2: The Dark World (2013) - 118 minutes
Captain America: The Winter Soldier (2014) - 120 minutes
Guardians of the Galaxy (2014) - 100 minutes
Ant-Man (2014) - 110 minutes
Phase II Sub-Total: 9 Hours and 31 minutes
All MCU Films leading up to Avengers: 21 Hours and 55 minutes of film to watch
At this point, if you have timed this to lead up to Avengers 2, you will need a sleeping solution as well so that you will not be fatigued and can properly enjoy the big movie. You'll also need time to put on your costume and make sure your Arc Reactor/Green Facepaint is working properly. Nothing crasser than a cheap Hulk.
Note: Assuming that Whedon's SHIELD show doesn't die, that's two seasons of SHIELD shows, at about 20 episodes a season, so, 40 episodes at 44 minutes each is 29 hours and 20 minutes of additional MCU footage. Bonus points if you also get in the 30 minutes of deleted footage from Avengers on a fan-cut, especially if you get fan cut versions of other films that include the shorts like The Consultant, a Funny Thing Happened on the Way to Thor's Hammer and Item 47. There will be other one shots in Phase 2. So perhaps we can sort of estimate this to be about an Hour, maybe an hour and a half, of non-film, non TV footage. Automatic "A" if you watch a fancut that puts all the events of the MCU in order. Protip: in the official timeline, IM, Hulk and Thor occurred all in the same week, so those movies and scenes would intermingle completely. The first scene in your film would be the beginning of Thor, WWII Captain America, but you wouldn't see the end of Cap until after the last part of Thor.
So... no, I will not be watching a marathon leading up to Avengers 2. If I'm with a bunch of Avengers hobbyists, I'm not above making a week of it, watching a couple movies a night with friends, but seriously, and it's a testament to how incredible the franchise is, Avengers is beyond the traditional marathon.
If I was a movie theater though, I would definitely host an all day marathon with all the MCU films, and would expect patrons to go in and out throughout the day.