The average lifespan for a dog post diagnosis of pituitary-dependent cushings is 4.5 years if they are treated and assuming they do not have other serious systemic disease. This is only an AVERAGE however so it does not always hold true.
It is definitely worth treating the disease. The only licensed treatment is Vetoryl. Some vets would claim that it is not as effective as Mitotane (Lysodren) but it still would be the first line drug in most cases. If it does not work, mitotane could be used, but it is a dangerous medication and there are risks with it.
There are other treatments for cushings but it really depends on what type your dog has (pituitary-dependent vs adrenal dependent) and the judgement of your vet as to what is best.
The treatment is reasonably expensive but generally works well. Some people have asked about Modrenal - this is the human version of Vetoryl. Even though it is one and the same drug it is not licensed for dogs and your vet would be breaking the law by prescribing it.
I hope this is of some help