I had a cat with a chronic/progressive illness. It was very difficult to know when "it was time", but I was helped greatly by the vet. When I'd taken him up because he "wasn't well" she said to me "if this was my cat, I'd euthanize him". I relied on her medical experience and knowledge of animals to have a better idea than I did as to when his quality of life had reached "that point". I had my cat's doctor to assist in the assessment of when it was time.