There's an excellent Youtube video put out by the Prostate Cancer Research Institute that looks at how PSA responds after various Prostate Cancer treatments: https://www.youtube.com/watch

The video mentions a study that was conducted on HDR brachytherapy treatment and found that the median length of time before PSA reached its nadir was 3 years. Also a PSA "bounce" is very common with all radiation treatments (a rise in PSA level before a return). I found the video very helpful.