Obviously that answer will be different for each chicken owner.
Some feel every breath is a gift as long as there isn't pain. Some cull at the first sign of illness for the protection of the flock as the weaker members bring disease to the stronger.
I personally feel that animals don't wrestle with these decisions. They only know if life feels good or it doesn't. Chickens either are well or they aren't. Diseases are either permanent or not.
Animals hide illness until they can no longer sustain an appearance of health to protect themselves from predators, so by the time you see evidence, it usually means they are further down into illness than you realize.
If treatment doesn't bring notable relief fairly quickly, it usually means their health is permanently compromised.
Typically, a slowly failing hen will sleep more and more, become weaker and weaker, until they grow too weak, lay gasping, and simply pass away.
If she is sleeping more than eating and scratching with apparent ease (versus desperation), it may be time.
Sorry she is not rallying. It likely is internal laying problems which are generally incurable (unless you want to spend a lot of dollars for a hysterectomy, and even then).
LofMc