Joss Whedon had already confirmed that the last Marvel movie we will see Loki in will be Thor: The Dark World but then kind of contradicted himself when discussing the potential future of Loki at Comic-Con. However, Loki himself (aka, Tom Hiddleston) has now confirmed that Loki will not be in The Avengers: Age of Ultron.
“Loki’s not in it. Here’s the thing: I don’t think there’s anything else Loki could contribute to The Avengers, narratively. Joss gave me so many wonderful things to do in that first film that we’d only be trying to repeat ourselves, and probably less well in the second one.”
Now this doesn't necessarily rule out any potential part of a Phase Three movie, but it is still pretty unlikely. I think both guys are right in trying to move onto more villains rather than have The Avengers just face Loki's mischief over and over again.