Parvo virus can certainly be a fatal disease due to the severe dehydration that occurs from the vomiting and diarrhea. So the length of time that parvo occurs depends on the treatment given and how quickly the dog recovers. At the vets, IV fluids are given, along with antibiotics and medication to control vomiting. And even then up to 25+% of puppies can die.
At home treatment can consist of subcutaneous fluids (which you would need to get from your vet and learn how to give from your vet). Pedialyte and other fluids are fine, but often (as you've found) the dog can't keep them down. And there aren't any OTC meds that will stop that vomiting.
Here is a good website with more information you might find helpful. There are many pages, so check out the right side of the page for more links: http://www.VeterinaryPartner.com/Content.plx?P=A&A=580
The other thing to consider is that there is something else going on with your dog. An obstruction (from something he ate like a toy, sock, etc) can certainly cause vomiting and diarrhea. He is certainly at the age for something like that. Pancreatitis
, although not very common in young dogs, can also cause vomiting, inappetance, and lethargy. I would continue to look for a vet that is willing to work with you and consider out-patient therapy, but I would try to get the parvo infection confirmed as treatment would be different if it was something else entirely.
Hope that helps!