This is never an easy decision.  However, there are some signs that can help you decide when the time has come.  It is important that you fully understand the disease or injury that is responsible for your pet’s ailing condition.
Some questions to consider asking your veterinarian:
Do I have all the information about my pet’s condition?
Are there any options for treatment or care? (specialist or referral centre)
Is there a chance my pet can recover, improve or maintain good quality of life?
How will my pet’s condition progress?
How Can I Tell If My Pet Still Has A Good Quality Of Life?
Does your pet still enjoy/participate in most activities that he/she used to?
Are there any changes in how your pet interacts with any family members?
Does he/she still enjoy eating and have a good appetite?
Does your pet seem to be in pain? (reluctance to move around, go for walks)
Is he/she able to go outside or use the litter box on his/her own?