{{ChiKat}} You know I just went through this with Brandy. The last few times I took her to the vet I expected him to tell me to let her go, but he didn't. The last couple times he told me if I made the call he would not have an argument. The day I made the decision she had fallen three times and gave me such a pitiful look. I knew I wasn't being fair to let her go any longer. My vet told me that her breed was a dignified breed and it wouldn't be right to let her lose her dignity. (he was super funny in describing certain tiny undignified breeds)
I know it is the most insufficient advice ever, but you will know when it is the right time. Ans whether sooner or later it sucks.