In what percent of patients who had prostate cancer treatment does the PSA keep falling?

I have had prostate cancer. Had my prostate removed and also my testicles, I do not know the percent of patients that the PSA keeps falling. I know mine keeps falling, and I am grateful because it means I have no active cancer in my body. I have been cancer free for 4 years bow. Last PSA 0.002. That's like nothing.