How long can my mother-in-law carry on with very little fluid and food. I've looked after her for 15yrs.

More than 15% of people have atrial fibrillation and live with it nicely as a chronic condition. When there are other co-morbidities of aging then likely it could contribute to a weakening heart pump. Kidney disease is of course chronic and will worsen leaving you with the sad decision whether to go with dialysis and other heroic measures which are in my humble opinion more a torture than anything else. You do not give us an age. You can, as an inactive elder, live with wasting and starvation a long long time, even with minimal amounts of food and fluid. Are you now considering hospice and palliative care?
Most nutritional drinks which others recommend are likely out with kidney disease. They are mostly protein and most sufferers of kidney disease are on protein limited diets. Though it is doubtful your MIL will take enough to make much difference one way or another.

Try giving your MIL some nutritional drinks such as ensure. She has to get some supplements.

She is pathetically thin, frail, confused and suffers from CKD and heart troubles yet you feel that her having to bear this alone with carers (not 24/7 I take it?) and family providing care is somehow better than being in a place where qualified personnel can see to her dietary, physical and social needs is somehow inferior? It broke my heart to place my mother in a nursing home and although I railed against the inherent limitations I knew that the care she received there was better than what I could give her myself. Sometimes there are no good solutions so you have to choose the least bad, and there is a lot to be said for handing a share of this terrible burden to those who are trained for it.

I’m sorry you’re watching this happen. My mother lived for three full weeks with no food at all and only what was termed minimal hydration. Her doctors advised us not to allow her to become dehydrated as that is a painful way to die, so she was given a little water every few hours, not enough to prolong her life but enough to ward off the pain of dehydration. I hope you’ve contacted both your mother in law’s doctor and hospice for guidance for this difficult time.