Probably all the time from now on. Not every movie can be titled "Avengers", even if they are a ensemble movie. So pretty much all the sequels, and even some of the origin movies, will have appearances by other characters. Some more and some less significant than others.First we have Captain America 3 transforming into an Avengers film. Now we have Thor 3, which features the Hulk as a co-lead and Spider-man with Iron Man making more than a cameo appearance.
How long is this pattern going to last in the MCU?
I personally dont mind it. I like that Ragnarok is basically MCU Episode XVII instead of Thor 3.