I don't have personal experience of a dog suffering kideny failure but I have seen a few at work. Like Jane says, it is not nice to see a dog at the last stages
As her disease progresses, the toxins will accumulate in her system and she will eventually go off her food as she will feel really miserable. I'm not sure if its the same in dogs, but in cats soemtimes they can get ulcers on their tongue as a result of the uraemia build up resulting in a lot of pain. To put her on a flush, she would need to be on a drip for at least 2-3 days with no guarantee that it would help. You would just have to assess whether it would be worth it. My old dog Brook developed hepatitis and the vet put him on a drip for 4 days. He was thoroughly miserable when he was in the vets(as I knew he would be) and as it turned out, I only got another 8 days with him before I had to have him PTS due to his liver failing rapidly.
You know her best so you will be best to judge how she is coping. I thought I wouldn't be able to tell when the time came as I would obviously be trying to keep him with me as long as possible, but I just knew. I'm sure you will too when the time comes.