I do not know the answer and wonder this myself...as my lab has bone cancer in his front leg. He was diagnosed 3 months ago...the way the disease is progressing (even with tons of anti-oxidants and vitamins and good foods...the tumor is much larger and he is starting to limp. I am guessing if the rate keeps up...he will go without massive suffering perhaps another 1-2 months? I will put him down when he no longer will use the leg-as that may indicate terrible pain. He is on anti-inflammatory drugs currently to take care of the pain. My vet told me that perhaps he had 4-6 months prior to suffering.