OP, I'm so sorry to read this - I lost both my dogs to cancer in the space of four months last year. In both cases, it came out of the blue and it absolutely broke my heart.
One crumb of comfort I can offer you is that for both my Ddogs, the end came very quickly, and up until then, both were almost as normal. I had no idea the older dog even had cancer - she went downhill in the space of one weekend and the tumours were only diagnosed pretty much at the last minute.
The vets told us that the most common cancer in dogs is very aggressive, hence they don't really recommend chemotherapy unless the dog is young - the treatment's not as stressful as it is for humans, but it only gives an additional few months of life. But they also stressed that they weren't oncologists, and were happy to have a consultation with an expert if I wanted. I decided not to.
As soon as we made the decision not to put my younger DDog through chemo (ffs, I have tears in my eyes even typing this) I spoiled him rotten. Let him sleep on the bed, fed him mince and healthy treats, took endless photos of him, spent hours just cuddling him. He died three weeks after the op to remove his cancerous spleen, very suddenly on a holiday that we were going to cancel until the vet said, Oh no, take him away and let him play on the beach and give him a nice time. So we did. He romped in the garden, chased his mate, lay in the sun. The next morning he wasn't quite himself, and when the local vets x-rayed him, they found another tumour, this time inoperable and spreading fast. We let him go, in our arms, and the thought of the happy, loved days he enjoyed right up to the end were a big comfort in the sadness that followed.
Sorry, that was an essay! The TL:DR version: you'll know when the time's come, but your DDog has no idea. Until then, spoil the heck of your DDog and take lots of pics.