Samuel L. Jackson was doing an interview with IGN when they asked him about his role as Nick Fury in the Marvel movies. When asked if he was going to keep on playing Fury, he informed the interviewer that he only has two movies left in his Marvel contract.
Looking at the future Marvel movies, he is already seen in the trailers for Avengers: Age of Ultron, but this interview was released on Feb. 11, so that movie was already in the can, meaning he has two more movies after that. The thought process is that he will be in Captain America: Civil War and maybe one of the Avengers: Infinity War movies, that that is just spit-balling.
Of course, Samuel L. Jackson is 66 and will be 70 by the time that the second Avengers: Infinity War movie hits screens, so he isn’t getting any younger. However, Jackson has really done an amazing job of seeming to stay young (maybe he has the same youth potion that Nick Fury has) so he could possibly go until they don’t need him anymore.
However, the idea is that Avengers: Infinity War might mark the end of The Avengers as we know them with The Inhumans starting the next phase of Marvel movies, since The Inhumans will likely be the Marvel Cinematic Universe version of mutants. Plus, there are a ton of characters who could take over leadership of SHIELD if Nick Fury is taken out of commission, including Captain Marvel, Skye from Marvel’s Agents of SHIELD or Maria Hill.
Anyway, here is the video where Jackson talks about it and makes it clear that he wants a contract extension and maybe a Nick Fury movie before all is said and done.