Excellent film Peter. Thanks for the reference. Maybe some optimism in sight at last.
Probably millions of years if the earth were left to its own devices and natural cycles without the intereference of the human animal which will undoubtedly be the cause of the end whenever it arrives.
Of course this question cannot be answered. After the end of the world we shalln't be here to see whether the answer was right or wrong. And perhaps Homo Sapiens will become extinct and the world will go on without us.
Man has been degrading the environment for thousands of years. But we have been given intelligence and we can make choices. This film by John D Liu gives some examples:
It's well worth watching.