Not everyone can hold it for three hours. After around the hour mark, you start missing a lot if you dip out for even a few minutes. I agree with my friend Matt Singer on a wide swath of things, but I disagree with his assertion of when one should/could go pee during Avengers: Endgame. There's a better spot about 15 minutes later. It's after the cut.
I respectfully disagree while at once saying it is a place when you can go, but I'd argue you would miss crucial development for a major character arc.
The far better pick is approximately 55 minutes from start. It might be a little sooner, we had a jerky start that offset my notes, so keep an eye out once you're past a breakfast scene.
Run like hell when you get to a scene with two Avengers where one says that something is "definitely Barton".
That is when you should get your 4-5 minutes of relief, right then. If it jumps to another location in the rain before you get up, you can still probably get a solid 3 minutes. Even if you're a hair late coming back, you don't miss much at all.
You really don't want to get up at all during the last two hours, but if you are just absolutely dying, you can make a break right when a car drives past after we arrive in New Jersey. You can, but I'd advise against it. There's some great stuff in New Jersey.
I politely disagree with Matt, but I'm downright taken aback at CNET's clickbaiting piece to the point that I won't even link to it. Likewise ignore GameSpot and ComicBook. IGN mostly agress with me, but man alive did they not need to spoil various details like character names. Irresponsible, the lot of them, to the point that I felt like I had to write this.
Go find CNET's thing if you want, it's a (mostly) free world, but be warned that it's factually incorrect about when what happens in the film. First, they flat-out get the timing wrong of when a scene happens, placing it "just over an hour" into the movie, when I clocked the scene they described as ending at around 39 or 40 minutes. On top of that, it wasn't even lunch! It was breakfast, the optimistic meal1! They also go much more spoiler-ish than anyone would want them to.
Trust me, trust Matt, trust no one else and come back Monday and I'd dare you to call us liars.
1 You'd only get this if you watch Patriot on Amazon. It's one of the best spy shows ever made.