There are so many factors, it's really a tough question to answer. If the seed is good, and you plant it where other seng is already growing, and there are no deer/turkey/squirrels/slugs to bother it, and the weather is good for 10 years, and there are no fungus or disease present, and no poachers find it, and they don't log out your woods, you could probably get 80% or higher.
But it's not likely you'll find a location like that.
I'm not trying to be a jerk, I just wanted to point out how many stars need to align to get ginseng to grow.