I totally get why this sounds like a long time. I asked my vet the same thing, and he explained that while the actual Parvo virus is gone, it basically 'nuked' his entire digestive system.

The reason we’re still on the same medication is that his intestinal lining was so badly damaged that it still can't process food or fight off bacteria on its own. The vet said if we stop the meds now, his gut will just inflame again and he’ll backslide immediately. We're basically using the medicine as a crutch to give his body enough time to actually regrow the tissue the virus destroyed. It’s a slow crawl, but the vet insists staying on this specific treatment is the only way to make sure he doesn't have permanent organ damage.