My eldest childhood dog had vestibular syndrome, we woke up one day and he was walking in circles/ falling to the side! We waited a couple of weeks and he seemed to be getting better, but one night deteriorated quickly and we had to get the vet to open as an emergency, where he was put to sleep.
I watched my other two dogs be put to sleep too, and it is one of the hardest things and decisions you will ever make. I do not regret taking them out of their suffering though.
We called it when it seemed they was suffering too much. Each one was a different reason, but when they was in pain or had deteriorated so far it would have been cruel to carry on was the right time.
My thoughts are with you.