Marvel studios/Disney will ride this horse until it is truly dead (and then maybe some more) and it's anyone's guess when that will happen if at all.
For me it's a small miracle that they have not produced a true dud.. sure, some movies made more money than others but in general they are well liked which is a small miracle in my opinion.
The reason for this is that they have followed two simple rules for every movie:
1. It is produced by people who understand and love the characters.
2. It is produced by people who are able to translate super-heroes into a story non-comic book readers can appreciate while still being true to point 1.
For me, it is a small miracle that no studio has realized this formula before. We all knew it.
As for the longevity, the closest parallel I can think of in movies is the James Bond franchise which has been in existence fifty years and going.