It's one of the most long-awaited movies in the history of movies.
Avengers: Endgame premieres tonight and people are freaking out about the movie being 3 hours and 2 minutes long (but honestly, it could be 8 hours and to me, it's not long enough).
So WITHOUT SPOILERS BECAUSE WE DON'T SUCK: there are suggestions as to when to run out to pee and run back in according to CNET:
When the San Francisco title card comes on screen
If you made the crucial mistake of not peeing before the film, you won't miss much if you duck out now. This Ant-Man segment is about 30 minutes into the film and is mostly just Scott connecting dots the audience already knows.
When Hulk is having lunch
This scene is just over an hour into the film, and while it's amusing, missing it isn't a dealbreaker (in fact, you may escape some cringe-y silly moments). The next 10 to 15 minutes after this scene is the safest window to empty your bladder.
When the New Jersey title card comes on screen
There are no good spots to pee in the last hour of this film.
We repeat, there are no good spots to pee during the last hour of this film. So if your bladder's at the end of its rope, pee now! This segment is your last decent opportunity for a bathroom break. While it's filled with nostalgia, you won't be missing any real conflict and it's fairly easy to fill in the blanks. Plus, if you pee fast enough, you'll probably catch the tale end of the segment.