k.a. 3-Ingredient Steak Sauce)" from Genius Recipes. How long does this sauce keep if left over? It seems like it would be nice to keep around in t...

The intensity of the mustard will fade within a few days, i’ve used colemans before and was surprised that happened. Just make what you will use within a week or so.
Especially since it takes maybe two minutes to mix together

While I haven't made this specific Roy Yamaguchi recipe, I've made similar soy-vinegar sauces (like Japan's traditional sanbaizu) and those can last several weeks, even months in the fridge.

That said, I typically try to use these things up in a couple of weeks, more for the principle of minimizing clutter in my fridge. "What the heck is this? How long has it been here? What am I supposed to do with this?" If I don't enforce any sort of discipline, there would be 20-30 small jars of hazy provenance.

In a restaurant this type of sauce would probably be made 2-3 times a week.

Anyhow, best of luck.