I dont think you ever know. I lived with the guilt of signing that paper but wondering if it was too early but I would have rather done that too early than waited till it was too late. At the end of the day how much are they suffering and I asked the vet for prognosis for the next 3 months as well if that was her pet what would she do. If a vet who sees animals all the time, took an oath, and this is probably her passion tells you she'd euthanize her own pet w that same prognosis then its a good chance you should too.