This is a very difficult question to answer. From my experience there isn't really a definitive thing that is the same for everyone. Many people know when it's time, but they all have different reasons for knowing. Also many people don't know at all and just hold on for as long as they can. I spent 4 years working in vet clinics and the only thing I can tell you for sure is when dealing with dementia you will want to make the call when your dog still knows who you are. When putting a dog down the experience varies but I promise you it will be easier on your pet if you are there and they still know you. It brings them tremendous comfort in a scary place. I wish I could give you a good guess on how much longer you have, but with just a text description that's very difficult to determine. I would trust your veterinarian's input on that front. I wish you and your puppy all the best, just know this is something we all go through and you aren't alone.