A lot more if she eats dry than if she eats wet food. If she exclusively
eats wet food, she may drink almost no water at all if she's healthy. I
saw a special on Animal Planet that showed desert cats living wild
(feral cats, in the desert) and they had no water at all - only lizards,
mice, and voles.

From http://maxshouse.com/feline_nutrition.htm

[quote]
Water Requirements

Water is the single most important nutrient necessary to sustain normal
function of all living cells. Water helps regulate body temperature,
cushion the joints and internal organs, digest food, eliminate waste,
lubricate tissue and allow salt and other electrolytes to pass through
the body.  The nonfat component of mammals contains about 73 percent
water (about the same amount found in canned food). Cats can lose nearly
all their reserves of glycogen and fat, half the body protein stores,
and 40 percent of their body weight and survive. However, cats are much
less tolerant to losses of body water.

Cats can withstand acute dehydration slightly better than dogs.  
Although cats can tolerate some depletion of their body water for a
short period, they must in the long run remain in water balance.  The
losses of water from the body must be offset by an equal intake of
water. At normal temperatures water is lost from the body via the lungs,
skin, urine, milk, and feces. At high temperatures, an additional loss
may occur via saliva, which is used to wet the fur and provide
evaporative cooling. The body gains water from "free water" present in
liquids and solid foods and "oxidation water" arising from the
catabolism of carbohydrates, fats, and protein.

The cat has evolved to obtain her water requirements almost entirely on
the moisture content in her food - inherited from her desert-dwelling
ancestors. Cats can live for long periods without drinking water when
receiving food containing 67-73% water but become dehydrated when the
water content of the food is 63% or less. Canned diets contain enough
water that cats consuming them rarely need to drink.  Daily water needs,
in milliliters, often are "guesstimated" as equal to the metabolizable
energy requirement in kilocalories or approximately 60 ml/kg.  Once the
diet is consumed, oxidation of nutrients produces an additional 10 to 13
grams of water for each 100 kcal of metabolizable energy.   Thus a 4 kg
cat consuming a 240 kcal canned diet containing 78% moisture will
consume 237 ml or 98% of its daily water need directly from the diet.  
Thus the cat needs to drink less than 1 oz. of additional water per day
whereas a cat consuming a 240 kcal dry diet needs to drink over 7 oz. of
water per day.   This can be difficult because cats are not naturally
big drinkers.  Feeding a canned diet containing 78% moisture virtually
guarantees homeostatic control of water balance in the cat.

The water content of the commercial foods commonly fed to cats varies
from 8% in dry foods to over 75% in canned foods; thus the amount of
drinking water required is affected substantially by the water content
of the food.  When fed canned food (80% moisture)  with access to
drinking water, cats obtain over 90% of their total water intake from
the diet, whereas on dry food, 96% of the total water intake is obtained
by drinking. The total free water intake (from food and drinking water)
decreases when cats are fed dry food only, so that the water to dry
matter intake ratio when fed on commercial dry foods varies from 2.0 to
2.8: 1 whereas on canned foods it varies from 3. 0 to 5.7: 1.  Thus for
any given dry matter intake cats have a higher water turnover on canned
than on dry foods.  (National Research Council [National Academy of
Science] Nutrient Requirements of Cats).

Diet moisture content is related to the observation that cats fed dry
food drink more than six times more water than cats fed canned food but
that much of this water contributes to fecal moisture so that urine
volume is lower and urine specific gravity higher in cats fed dry food.  
The urine concentration of all solutes, including potentially
calculogenic crystalloids, depends on urine volume.  Cats increase
voluntary water intake when fed dry food but not in sufficient amounts
to fully compensate for the lower moisture content of the food.  In a
recent study, cats consuming a diet containing 10% moisture with free
access to drinking water had an average daily urine volume of 63
milliliters (ml). This volume increased to 112 ml/day when fed a canned
diet with a  moisture content of  75%. Urine specific gravity was also
higher in cats that were fed the low-moisture food.  Decreased urine
volume may be an important risk factor for the development of
urolithiasis in cats. Diets that cause a decrease in total fluid
turnover can result in decreased urine volume and increased urine
concentration, both of which may contribute to urinary tract disease in
cats.  Several studies have shown that dry cat foods contribute to
decreased fluid intake and urine volume.

Homeostatic control of water balance in cats differs in some important
respects from that of dogs  Cats make less precise and rapid
compensatory changes in voluntary water intake than dogs in response to
changes in the water content of their food.  Similarly, their
compensatory drinking response to dehydration due to increased
environmental temperature is less effective than dogs.  This apparent
weakness of the cat's thirst drive to respond to changes in her state of
hydration has led to the conclusion that feeding canned food assures
adequate hydration at all times.

In addition to ensuring adequate hydration, a high water turnover helps
eliminate crystallogenic substances before they grow to sufficient size
to interfere with normal urinary function.  This is a very important
consideration for male cats.   Cats that cannot urinate for more than 24
hours due to urinary tract obstruction can die from acute renal failure
and/or severe damage to the urinary bladder.  In addition to the removal
of crystals, benefits of increased water intake include dilution of any
noxious substances in urine, and more frequent urination to decrease
bladder contact time with urine that may reduce the risks of urinary
tract disease.  For that reason, canned diets are usually prescribed as
the first-line therapy for feline lower urinary tract disease.

[/quote]

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thinks you'd make a nice sandwich.
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