I think there's merit to the idea of releasing every two years, running off the momentum from Phase I to carry the franchise. I think there's more merit in continuing to build up the Avengers into event movies. Not only do you get to create bad guys that make Avengers 2/3/whatever independently compelling, but if your build up is just as good or better than the previous phase, then you keep all the momentum you had, and add more. Now it's not just about what will Cap do next, but what will Cap do with Black Panther, and how will they stand a chance against Ultron?
I think 3 years is ideal, at 2 years, you're just maintaining, and will lose some people since you don't have enough time to make the concept of Avengers 2 seem bigger than the concept of Avengers 1. At four years, unless your buildup is pretty frikkin epic, you'll lose people's attention. At three years, you not only have a normal amount of time between sequels, but you also have time to build it up so it'll still be an event movie and get event movie money and excitement.
You also have time to let it breathe, which you need so you don't rush out something crap, since you don't have a prewritten story like TwiPotter Games. Two years simply isn't enough for the Avengers franchise to continue to grow.
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