I'm very sorry. The best I can tell you, is that you will probably know. I base my decision on whether I think they are suffering or not, whether the suffering is temporary and is likely to possibly improve with treatments or therapies, or is likely to just continue no matter what I do, and whether they are eating, drinking, doing relatively normal chicken things or not, is quality of life good, fair, poor. I do know that when they act unwell, they are usually very, very unwell, otherwise they hide it really well. If they stop eating and drinking, are showing signs of not being really uninterested in normal chicken things of all kinds, that is when it's probably time. Sometimes I will leave them if they still have some fight, just depends on the overall picture. The more birds you have, over time, it becomes a clearer decision, as you experience what their decline generally looks like. There is no one right answer for them all. If, in your opinion, she is suffering needlessly and her quality of life is poor and not going to improve, then it's not a wrong decision. Sometimes it's the biggest gift you can give them, despite how hard it is for you. I read an article somewhere about end of life decisions for other pets. Research showed that people who had more experience with health and age problems in their pets over time, tended to make the decision sooner rather than later, the more times they'd had to deal with it. Not because they didn't care, but because they knew how it was going to be. I've had to make some heart wrenching decisions with pets, seizures, cancers, etc. it is never easy. But watching them suffer needlessly isn't fun, it's stressful, and isn't really fair. It's not black and white, there are many shades of gray. If we really listen to them, they usually let us know. 
