When should I put a dying kitten to sleep?

I have a kitten who has feline leukemia and it’s in the final stages of the disease. We are all devastated. She is not in any pain, but over a few days she has not eaten or drank any water and she has lost a lot of weight. She is aware of what is going on; her eyes are open quite wide and she responds to the things going on around her. But, she is very weak and has been foregoing trips to the litter box and is peeing where she is: maybe out of weakness or loss of control. But, she seems to enjoy being on the bed with me and with her sister kitten and I can hear her purring sometimes, even just today. I love this kitten and I can’t tell when to make the decision that life is bad enough for her or to let her have a few more days in peace. It’s tough to tell what she may be thinking or feeling, although I don’t get the impression she is in pain, though she might be fuzzy-minded.