Prognosis for a Puppy with Parvo?

My pup's only 45 days old; he is a black Dachshund. He's been diagnosed with parvo. 2 days later the vet immediately started the treatment. On the second day the vet was not able to find the veins as it was very difficult, so they have given it through the skin. I just want to know whether it is useful and effective to be giving the fluids through sub skin layer. How much time does it take for him to get back to normal? He had bloody diarrhea and vomiting. What are the signs that my pup's health is improving?