Quoted from CrazyLevi:For me it was when he really couldn't stand anymore. I mean, he wasn't in any pain yet really, he could still see...but I mean when you can't fucking walk across the kitchen floor without slipping and falling it just seemed like the right time. At that age you know it's not going to get any better.
That is a good indicator. I lost my last dog in my arms on a Father's day, but she had a good long life and similar symptoms, though the walking wasn't quite a problem yet. I lost a cat who went through a heartbreaking 6 months of slow deterioration due to bone marrow cancer at 10 years old. I lost another cat instantly to a visiting dog which was tramatic to say the least... and several other animals to old age. Slow or fast, it's all terrible, but it's still worth all those years of joy. Cherish the time you have left, which goes for pets and people.
The key decision is when the suffering is too much... whatever that means to you and your vet. It's an awful thing to deal with.