My greyhound, Foxy, suffered with osteoarthritis aged 13+ and did well on Galliprant although the efficacy plateaued after a while. She had Seroquin as a joint supplement and, latterly, Tramadol for extra pain relief.
I can only say that, from my experience, I was led by Foxy as to how much or little exercise she would have. On good days she tottered happily round the village, meeting her adoring public and sniffing every scent mark along the way. On bad days we just went slowly along the drive and back, playing "catch the biscuit" as a distraction. I honestly think you will know when her pain level exceeds her wish to move about and engage with life, as you clearly love her.
A word of warning, when Foxy was ready to die, she became quite restless and wanted to keep walking, despite the pain. Apparently it's not uncommon but I only found that out later and probably kept her going a day or two too long. We wish you well and understand your distress.