How long does a new dog food cause stomach upset for the dog?

I can find all kinds of articles on how to switch dog food, but nothing says how long stomach upset lasts if you don’t do it properly. We are dogsitting and the owners didn’t leave us enough food. Not being a dog person myself, I didn’t think to save some of the food when we started to run low (but now that I think about it, we would have started the process soon after the dog arrived to do it properly). Anyway, it’s been four days since we started the new food, and tonight the dog had a bit of the runs. Only once, but I feel bad for her. How long will the diarrhea last, and at what point is it a vet-worthy concern? I tried several different stores before settling for something that seemed as close as possible to the food they left me. and WOW dog food is expensive! The owners will be back in two days, and even though I know we did the best we could, I am worried they’ll be upset. :(