What duration androgen deprivation do you recommend with salvage pelvic radiotherapy for pelvic node relapse after prostatectomy?  

6 months ADT, 2 years ADT, 2 years ADT + abiraterone/prednisone seems reasonable depending on the circumstances. For example, early pelvic node recurrence for pT3b Gleason 4+5 would be a higher risk than a very late recurrence of pT2 Gleason 3+3.

Agree that there is some nuance in patient and tumor features, but ultimately this is cancer that has shown it can spread at least to lymph nodes so I generally aim for 2 years and at least discuss Abiraterone. If toxicity is high then can consider cutting short Things that push me to shorter cours...

At least 6 months, preferably 18 depending on tolerance 

One option may be to assess the androgen sensitivity of the recurrence by using Decipher, provided the area is amenable to biopsy. The result would offer the potential to tailor ADT to the patient.