How do you know when it's time to let go?

We have just lost Moses, our beautiful nine-and-a-half-year-old Pyrenean Mountain Dog, to bone cancer. Moses was diagnosed with bone cancer last Friday and he was put to sleep on Tuesday. The house is really quiet without him and we are very upset.
We had known something was up for the last month or so as he was always very happy to charge around but had recently slowed down on walks. I phoned our breeder, who has been wonderful and has offered advice throughout Moses’s life, and she recommended a change of diet. Then after two weeks, he began limping, so we took him to the vet, who diagnosed arthritis and put him on an anti-inflammatory, which seemed to have a positive effect and he was bouncing around again. However, this only lasted for a week or so and he began to slow down again. We knew something was very wrong as he had never failed to get over-excited at the sound of the car starting-up before, but he was now very still.
We took him back to the vet on Friday, who found a swelling on his leg, x-rayed him and told us of his diagnosis. Having googled survival rates, we thought he may have about a month left as he was still eating masses and giving his paw. Our breeder was very sympathetic and quietly said this may not be the case and that we would know when it was time for him to go. Our vet had also said we would know when the time had come, but actually we weren’t really aware of what those signs were. On the Tuesday, Moses was panting a lot and he was wanting to spend time on his own, when previously he had always been around the family, and it was this distance that made us realise it was time.
Moses has had a very healthy life – he hadn’t actually had arthritis – and although the larger breeds are more prone to cancer than smaller dogs, we had not realised quite how aggressive bone cancer is. The vet told us that by the time 90 per cent of bone cancers are detected, the disease has already spread to the lungs. We are obviously very upset at having lost a member of our family so quickly, but very grateful to our breeder and our vet for their wonderful support.
People always say ‘you will know when the time comes’, but I am unsure that we did really know. We weren’t aware that panting was a sign of pain and we weren’t aware of how much pain he was in (we have heard bone cancer is the most painful form). Has anyone else felt this way when their dog was desperately ill? We would not have wanted to prolong his life if he was in pain and hope we have not caused him unnecessary suffering, and I was wondering how others have ‘known’ when it is time to let their dogs go? Thank you in advance for your thoughts.

Jess, by phone