I've been a part of this forum for several years. I read a lot of stories of fellow pet lovers having to make this difficult decision. I always thought I could not do that. Then when my kitty Taz became ill I turned to my PT family and asked the same question you are asking now. They said I would know when it was time. But I didn't. I kept asking Taz to let me know when it was time.
After having to give him subQ fluids for two weeks I took him for a check up. He weighed 6.7lbs (he weighed in at 21.5 at his heaviest) and the vet basically said there was nothing more to be done. That's when I knew it was time to let him go. He let out a big sigh when they gave him the first shot to relax him. Oh boy, the tears are flowing again and this was almost 3 years ago.
Anyway, even though you don't think so now............you will know when it is time. I can't tell you how, but you will know.