I think if you know your animal well, you know when they've had enough. When Nougat had flystrike, I kept going with her treatment when maybe others thought I should let it go (no one said this btw). BUT her eyes still had a sparkle in them. WHen she lost that sparkle I know it was time ot let her go. The same for Patch. I could see from her eyes she was still enjoying life (and being with her Bobby).
In both cases, I had taken the decision to pts, but the bunnies beat me to it! Nougat died naturally on the vets' table and Patch in the night ( I was going to take her to the vets the following day).
Each case must be taken on its own merits I think.