Depends on all your other health factors. If you are a reasonably fit, healthy man, with treatments, one can expect to live out a normal lifespan and eventually die of old age. If a man is obese, has high blood pressure, heart problems, etc, even with treatment might drop dead tonight or pass away in five to ten years.
The thing is, prostrate cancers are the most treatable ones. It is also one of the SLOWEST. There are quite a few options from surgery to radiation therapy, to doing nothing. I'm 64 years old. I was diagnosed with stage three prostrate cancer about four years ago. Because I was not a good candidate for surgery, radiation was my recommended treatment. About three months of that and it appears my remission is as close to 100% as they can measure. If I had been diagnosed at stage one, my doctors might have recommended I do nothing because I'd die of a lot of other causes long before my cancer even proved bothersome.
Remember: Slowest growing and most treatable.