There are a lot of variables. I like the hydration calculator at this site:
http://nutrition.about.com/library/blwatercalculator.htmIt takes into account one's height, weight, climate, exercise, etc., and then calculates the amount per day. It also warns people with adrenal or kidney problems to check with a doctor before trying this, and reminds people NOT to drink it all at once, or it can deplete electrolytes and cause serious effects.
Foods contain liquids, too, so should include milk, juices, sodas, soup, etc., in the total. If I have another drink, I'll just empty an equivalent amount out of my bottle.
He didn't know why, but a lot of people don't feel normal thirst, according to my doctor. Over the years, there have been several docs who couldn't diagnose me, but who were able to pass along helpful suggestions to feel better. I'll always be grateful to them!