Here's the data on adjuvant chemo with primary external beam radiation therapy. Really very little benefit for the toxicity. I can't recall any studies of it being used in the salvage situation./pcnrv.blogspot.com/2016/08/docetaxel-with-primary-radiation.html
The RO and you should decide how long to run the ADT for. In the salvage situation, we know that at least 6 months, and probably at least 12 months has a significant benefit. Beyond that, it's really a matter of clinical intuition, and the RO is the one who would have that intuition based on his treating many patients with adjuvant ADT. It also depends on your husband. If he tolerates the SEs well, he may be more inclined to stay on it longer - up to two years. There's also some trade-off with radiation dose-- if he's getting, say, 72 Gy of SRT, he may be able to get by with lower duration of adjuvant ADT.This is something you should arrive at through shared decision making./pcnrv.blogspot.com/2016/08/combining-androgen-deprivation-therapy.html
Knocking his PSA down to undetectable with the adjuvant ADT would be ideal. But remember that PSA is a gross indicator and tells you nothing about
what is occurring on the cellular level. Some of the cancer cells within a given man are inherently hormone-resistant than others, and may take longer to be killed.
One doesn't stay on adjuvant ADT. Eventually he stops it. If it didn't finish off the last of the cancer, PSA will slowly rise again. It is considered recurrent if it rises over 0.2. It would be quite some time until anything is visible on scans.