the True Nephelim all died but their blood was still in people including those that went on the ark. From my understanding the dead nephilmem became the "evil spirits" in the world trapped between here and heaven. They also were travelers which is why the stories are WW and the stories that go along with them they are almost ALL the same. Slight name changes but the PLot is all there.
how long was what? Before the rise of Giants after the flood? What is your question?