I am so sorry to hear about Grady. I am a veteirnarian and will do my best to help.
I always hope my cats live to be 20, and I have seen 30 year old cats! However, of my dozens of cats, most have lived to be 15 to 18. One made it to 19, and one died of cancer when he was only 10.
If removing his leg cures the cancer by getting rid of the tumor, he could live for many years to come. It is also possible that the tumor could have already spread to his liver, or lungs, or lymph nodes, and that he may not have that many years left with or without amputation. If you are thinking about having the surgery done, I would consider a chest Xray and basic blood tests, if not an abdominal xray to check for this kind of spread.
If you decide to have his leg removed, you will be amazed at how well he does. Dogs and cats do not greive for lost limbs the way we do, and quickly adapt to life on 3 legs.
Please let me know what else I can answer. Rebecca
Let me know what else I can help with or answer. Rebecca