Just for you, I got out my handy dandy dog care book:

Let's see : loss of appetite, loss of appetite. Oh, yes! page 161:

It says that if your dog has any of the following signs, you must take him to the vet within 24 hours:
- Fever
- Pain
- Weakness
- Cough
- Vomiting
- Diarrhea
- Increased water intake
- Decreased water intake
- Increased urination volume or frequency.

If the dog is less alert you must also go to the vet.

If your dog doesn't have any of those signs, then you are supposed to follow the "home treatment":

This is from The Dog Care Book, by Sheldon L. Gerstenfeld, V.M.D.
"Try adding some brewer's yeast or garlic salt to the food. They add "zing" and may coax the problem eater out of it's problem."

So - if none of those signs are present, then I would think it's safe to wait 24 hours and see if Rio's appetite improves!