Do not wait longer than 1 day. Plain white rice and lots of water (try mixing it with low sodium/ no sodium chicken broth) is option 1. Be careful to check what the water intake is to be able to provide that info to your vet. Remember that this is simply a symptom and while it is very unpleasant, the reasons for it might be quite serious.
If you have abruptly changed their food then this might be a consequence. It could also be related to something your dog ate at the park (food he/she found) , maybe chewing someone else's treats to which they might be allergic or even sniffing the wrong dog who has a stomach bug.
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