According to this site Cap got the most screen time
Captain America: 37:42. He squeaks by Iron Man, which makes sense, since much of the film’s internal debate about selfishness versus selflessness occurs between Chris Evans’s Cap and Downey’s Stark. He shares substantial scenes with almost every character and is one of the last faces we see on screen, as he rides off on his motorcycle.
http://www.vulture.com/2012/05/how-m...enger-get.html