How Long Can A Person Sustain With Only Fluid Intake?

My mother is 94 and suffered a stroke 18 months ago, she has stopped eating, but is having some liquids, she was having trouble with fluids so they put her on thicken fluids. All medication has been stopped and this was 16 days ago. She has days where she sleeps all day and has nothing by mouth, and the next day she is talking and having fluids? How long can this go on?