We had 2 dogs, Tosh, a springer/cocker mix was 14 1/2 and had to put her down 2 weeks ago, she got sick a month before and was in kidney failure, given medicine, and then couldn't walk, stand up, it was time. The other dog, Kelly, a lab mix is 16 1/2, doesn't have the best appetitite, give her table food, whatever she will eat, some days she eats good, other not so good, she's on medication because of incontinence, she doesn't hear or eye very well, but she's comfortable, still excited to see me come home from work. Sometimes stumbles and falls. But she seems happy and she's doing OK. When her quality of life is not there, then it will be time. I think most of the time you will know when its time. You will see it in their eyes. Its a hard decision to make.