Oh tiggy2, I'm so sorry that Tus is having such a rough go of this. It's heartbreaking to see our furry friends so sick. I really can't tell you how long to keep going, it's such an individual thing that only you and Tus and the vet can try to figure out. I can tell you about my own similar experiences.
My kitty, Aztec, developed diabetes almost 7 yrs ago, but it was probably a month or 2 until it dawned on me that something was up. Took him to the vet for blood work, and before I got the results back, he crashed (vomiting, wouldn't eat, hiding in the closet). Off to emergency we go, and it turns out he's developed diabetic ketoacidosis (DKA), a life-threatening complication of untreated diabetes. He was knocking on death's door and needed intensive 24hr vet care for a week, plus a feeding tube, before he was able to come home. He was pretty miserable, hiding in the closet, exhausted and scared for several weeks. It was 3 months of tube-feeding every 6 hrs before he ate on his own, and most of my friends/family thought I was insane to put this much effort into saving him. But he pulled through! And now, at over 16 yrs of age, he is the happiest little love bug and the joy of my life.
His brother, Egypt, didn't make it to his 12th birthday. One day, he just stopped eating. When an appetite stimulant didn't have any effect after 24 hrs, he also went to emergency. An ultrasound showed a dangerously blocked gall-bladder and he was scheduled to have it removed that night. There had been no other evidence that anything was wrong. When they removed his gall-bladder, they also biopsied his liver/pancreas/intestines and put in a feeding tube. Biopsy results showed moderate IBD, which was a shock to me because he'd never had any symptoms (no diarrhea, no vomiting, good appetite). I was hopeful: IBD wasn't supposed to be fatal. But for him it was. His decline was steady and rapid, despite being on a cocktail of various drugs and supplements and forced feedings. 6 weeks after he first stopped eating, he gave me a look and a pleading meow that will haunt me forever. I couldn't get him to the vet fast enough, for the last time. I don't know what it was specifically that he couldn't overcome, whether maybe he had developed intestinal lymphoma (a potential off-shoot of IBD) or what else had been brewing inside his little body, but we'd come to the end of the road. I have no regrets over what happened, and instead look upon the whole thing as an amazing learning experience, with Egypt as my teacher. He taught me so much about love and life and death and for that, I will be forever grateful.
My heart goes out to you. Try to spend some time with Tus that aren't about her being sick, but just the 2 of you, sharing a peaceful moment, no thoughts of the future or the past. Focus on the sensations of that singular point in time, the warmth of her fur, the rise and fall of her chest as she breaths. Feel your love for her, and nothing else.