The immunity lasts at least 3 years but may even last the rest of your dog's life. After 3 years, you will need to get the parvo vaccine boosted. Your dog is still susceptible to the other viruses ...
I believe over-vaccination has caused my dog to have these GI issues, but neither vet will confirm or deny it. Now, my dog only has gross bloody diarrhea when she eats something outside her diet ...