When is it time to say Goodbye

I am wondering when it is time to say Goodbye to a beloved pet.  I have a dog who is 15 years old.  She is part shepard, part golden retriever.  I know 15 is a long time for a dog to live but she still seems so happy.  She is definitely slowing down these days and sleeps a lot but I just don’t know if it is time to say Goodbye.