Chris Evans stated in a recent interview:
“They may wait until 2014 until they release the next 'Cap'. Marvel has a lot of balls in the air. They aren’t going to cannibalize their films.
Out of those six films, if I pop up in one of those [other films], it counts as part of the deal. So if they needed me in a third 'Cap', and I say, ‘F--k you, give me $30 million,’ well, they want to avoid that.”
Geek Tyrant reports:
"Evans is contracted to appear in six films altogether, so that means there will likley be three Captain America films and three Avengers films"