Since Infinity War is starting to close this era of the MCU, it’s impossible for the movie to do everything it needs to if the running time is under two and a half hours.
As for 2019’s Avengers 4, Russo said that since they haven’t started shooting that yet, it’s hard to say what that movie’s running time will be, but he imagines Avengers 4 will likely be similar in length to Infinity War since it will also be packed with a lot of characters.
Joe Russo later acknowledged to Collider that he, his brother Anthony and the rest of the creative team have to “go big” on Avengers: Infinity War, as they have to take 10 years worth of Marvel storytelling (including “disparate” characters, themes and motivations) and pull all of that into a “unified narrative.”