DISCLAIMER: I'm not a vet and I have no veterinary or medical background whatsoever. This information on Cushings disease in dogs is not meant as a substitute or replacement for veterinary advice. It's meant for educational and informational purposes only, as a starting point for discussing the diagnosis and treatment of canine Cushings disease with a qualified vet.
We can only do our best. Learning to manage your dog's Cushings disease, along with the other issues that can crop up, can be a challenge - and it really helps to be able to talk to others in the same situation. Here is one great support forum for people with Cushingoid dogs.
At the time of writing, my dog is 15 and a half years old. It has been over one year from diagnosis and he appears to still be going strong. His Cushings Disease is not being managed at the moment due to his other health concerns, which are the more pressing issues right now. The only thing we're doing is providing him with supportive supplements for his liver (specifically, Zentonil and Marin, which are basically SAMe and VitaminE/milk thistle, respectively). His symptoms are undoubtedly progressing, particularly the excessive drinking and the weakening of his back end. And yet he continues to be happy and active. He doesn't know he has Cushings or any other disease for that matter, so he keeps enjoying life to the fullest.
Apparently, canine Cushings life expectancy is approximately the same, whether the dog is treated or is not. Many senior dogs have concurrent health problems, so it becomes a balancing act of deciding what's in the best interests of the dog to treat. The reason for treating Cushings Disease is primarily to provide a better quality of life for your dog for whatever time he has left.
More on Cushings Dogs:
Symptoms of Cushings Disease in Dogs
Treatment for Cushings Disease in Dogs
Cushex Drops Reviews - Did It Work for Our Cushings Dog?
*Canine Cushings - Life Expectancy