When to put a dog down?

Whoever's dog it is should make the decision.  You will know when you think that you have done everything you could and putting the dog down is the last thing you can do to help.  Some conditions are more treatable than others, obviously, and this will dictate some of the decision making.  If you feel you have a good vet than get their opinion as far as the dogs condition and internal organ functions.  If the dog is in obvious pain and yelps when you move its joints, and anti-inflammatories are no longer working, than I would say sooner rather than later 

Its the toughest thing I've ever gone through, but you did the right thing in getting a pup before you put the other one down.  I didn't do that and have put off getting another dog because I struggle with the thought of having to go through the whole thing over again.  Some people have an easier time with this than others.  I felt, like many, that my dog deserved every effort that I would have given any other member of my family.  Many, including my soon to be ex-wife, thought I was crazy and just throwing money away, but that was my decision, because he was my friend and my responsibility. 

I had a hole dug and made a casket for him, so that part was done, which made it easier when the time came, and doing this part right was the last thing I could do to honor him for what he had given me through the years.  The vet met me in the parking lot as well.  Spend as much quality time with your dog as you can, and you will know when the time has come.  Its tough cleaning up messes in the house all the time, but it goes with the territory.  You did it when he was a pup, so just grin and bare it now.