Yours is a hard decision. I had to deal with a long slow decline on my cocker mix, couldn't really tell when was the right day.
It's tough because some days he's fine and jumping on the furniture and playing, other days, he's loafing around sleeping all day. But he doesn't mess in the house at all, which is good, because some dogs older dogs have a tough time holding it in. My lab border collie mix whose about 12 now is having a tough time with that. We have to be really careful with him. But lately the cocker has only been picking at his food and refusing to eat. His situation isn't dire yet, but it's also made us start to think about the inevitable.