Billions for sure - we know because the natural reactor at Oklo 1.7 billion years ago is still very much detected. https://en.wikipedia.org/wiki/Natural_nuclea

Billions for sure - we know because the natural reactor at Oklo 1.7 billion years ago is still very much detected. https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...