I would try to give it at least for 7 - 10 days but if you do not see any improvement at all by day 5 then I would say to stop, it isn't working. But if you even the slightest bit of improvement keep going the full course.
Yes, I would mix it fresh each day. When my guys were sick, I would mix it up in a gallon jug and refrigerate it. It will seem like a waste though because you know the little one will not drink a gallon per day.
Does she still seem to be doing OK otherwise, like eating, activity etc?
LOL.....yep, I learned quickly too you always need a little hospital area. LOL