https://www.amctheatres.com/movies/avengers-infinity-war-45873
I think this is the first credible source for a runtime. That would easily make it the longest in the MCU.
Avengers: Infinity War
An entire universe. Once and for all.
An unprecedented cinematic journey ten year in the making and spanning the entire Marvel Cinematic Universe, Marvel Studios' AVENGERS: INFINITY WAR brings to the screen the ultimate, deadliest showdown of all time. The Avengers and their Super Hero allies must be willing to sacrifice all in an attempt to defeat the powerful Thanos before his blitz of devastation and ruin puts an end to the universe.
- PRE-SHOW AND TRAILERS RUN FOR APPROXIMATELY 20 MINUTES BEFORE THE MOVIE STARTS.
- 2 HR 36 MIN
- PG13
- OPENING APR 27
- ACTION
I think this is the first credible source for a runtime. That would easily make it the longest in the MCU.