How long does it take to rehydrate an elderly person?

I have an elderly relative that has dementia very frail doesn't eat much or drink some days. She has a lot of leg pain so that is the reason. The last couple days she has slept a lot & hasn't drank or ate much. Also she doesn't have much urine output, she slept 10 hours got up & just went a small amount. Urine is a dark yellow, a little lighter tonight, but when she pees it still isn't that much with all she drank. I'm sure the body is using it, and that is why. I called the dr. he said not to worry, just push fluids the next couple days, and it will pick up. She was more alert today did drink a lot more, but only about 100 liters all day. How much will she have to drink to get out more urine to a healthy volume again about how many days I mean? Her skin on her forehead doesn't tent when you pinch it together, she doesn't have diarrhea or vomiting, and she doesn't have sunken eyes, the corners are a little, but not the eye itself.