The fall of the Roman Empire took a couple hundred years. The fall of the British empire took 20 years. The way technology has sped things up, things could happen very quickly. Your timeline seems pretty reasonable.

That being said, the end of the fourth turning and the beginning of the first turning can overlap a bit. With more people returning to the land and practicing regenerative farming/Permaculture along with the increasing adoption of bitcoin, some people might be intentionally getting their own individual first turning.