I'm Sorry to hear this.
Having my cat put to sleep was the hardest decision I've made regarding a pet. My Tabitha wasn't eating, lost a large amount of weight. The vet gave her a Vitamin B12 shot, and saline. It helped for a couple weeks, then she went back to not eating. I took her back and the vet told me that the shots were only temporary, so I had her put to sleep that day. She was gone in seconds, I don't think she would have lasted another day.
Cherish the time you have left with her, she will let you know the right time.