I think you need to continue with PSA tests for life - it can come back anytime sadly, but frequency will reduce eventually to once a year or even less after some time.
You are still on hormone therapy which drags the reading way down. It's the readings when you come off that which are more interesting. I believe providing it doesn't increase by more than 2 over the lowest reading (which is usually some months after RT), it's regarded as having worked.
Often people stay on HT for 18-36 months after RT. However, I was reading some research papers on Whole Pelvis RT (as that's what I'm being recommended), and they are suggesting the best results come from getting PSA down to 0.1 before treatment, and then nothing is gained by remaining on HT after treatment. I will be asking my consultant about this, because I want to be off HT ASAP, constrained by not significantly compromising my life, obviously.