I think you know when the time is right. When our cat Oscar got cancer, she was gone a week and half after we were told. She had a massive growth in her chest, and it was obscuring her wind pipe, so she could not eat properly. We did everything possible, and when the time came, she had spent the morning at the vet's. The vet had told me first thing that morning she was ready to go, so Mr Spudnick and I went in prepared for the worst.
When we got there, she was quite active, and certainly, physically, did not look ready! So the vet advised us to leave her there and come back in the evening, and if she was still the same, we might get one more night with her. But when we went back, she had gone downhill, and she looked at us, and stretched her paw out to us. She looked peaceful, and ready to go. It was very hard, but there was nothing more we could do. She was 15, and had a brilliant life.
Our pets communicate with us. Listen to them, trust your heart, and you will know.
I am sorry for your predicament Hanrahan. I hope you are ok.