Sorry to read that Ian, surprised they did the surgery prior to the bone scan presumably they thought with a relatively low PSA figure the cancer won't have spread.
Looking at this positively you have no cancer in the prostate because it's gone. The HT should flatten the bone mets and seminal vessel prior to the chemo which will shrink the tumours.
As far as your question about how long you've got no one knows. I was given 2 years in August 2018 and after HT, Chemo and RT my PSA is undetectable although I realise this won't last. There are many of us on here holding our breath until the next PSA test.
Good luck with your ongoing treatment.