I always say when the tail stops wagging and the smiles go away it's time... It sucks worse than just about anything.
^^ This^^
I have a dog with degenerative myelopathy, which really is a terrible condition. I thought she would have been ready a long time ago, but I give her credit, her eyes still have that spark, still plays ball (albeit from a sitting position) and I am the one that has to fetch. She has lost control of her bowels, but can still control her bladder. She can't move though, other than to crawl a little bit. I ended up getting a bunch of those black carpet runner matts with the rubber underside. When I get home, and before I go to work, first thing I have to do is hold her hind quarters up so she can relieve herself. roll the matt up, take it out on the deck, hose it down, run a push broom over it with some dish soap, rinse and let dry.
On the other hand, I had to put down another one of my dogs last Friday, kidney/liver failure and that came on quick (3-5 days). It's always tough.