One of the most difficult decisions a pet owner has to make is when to say goodbye. The best thing to do is assess your pets quality of life and do what's best for your pet. Please find an assessing quality of life form in the attachment below to help you figure out if it is time;
https://vet.osu.edu/vmc/sites/default/files/import/assets/pdf/hospital/companionAnimals/HonoringtheBond/HowD oIKnowWhen.pdf