Always the more the better when it comes to water fasting. You will be doing great things while drinking just water, but (and there's always a but :) )
Try not to over do the water because anyone who drinks too much water can get water toxic syndrome. A gallon sounds about right. I drink about a gallon if not a gallon and a half a day while fasting with water and I have never had anything wrong with me.

I am on my 10th day today and feel great!

Keep up the good work and remember that too little can cause dehydration and too much will become water toxic. :)

Enjoy the water!!! :)

Warmest wishes,
IvanaB.Thin

this i found on curezone:
HOW TO WATER FAST:

You can fast from 1 to 40 days.
Try to drink 2 liters of water or more per day.
The ten day Water Fast has become a recommended
number of days. Ten days on water will cause the same
weight loss as 30 days on juice. But Water Fasting is far
more difficult, especially if you have a fast metabolism.
Water fasting cleanses the body aggressively removing
toxins rapidly. Water Fasting can be more beneficial
than juice fasting in combating more persistent forms of
cancer, cleansing the tissues more aggressively. Water
fasting demands mental preparation, the less pressure
and responsibility you have during a Water Fast the better.
Think of it a holiday away form the normal patterns of living.
Some recommend that the week before your fast, you drink
fresh juices and eat mostly raw fruits and vegetables to
cleanse the body so that the detoxification during water
fasting will be less aggressive. Water Fasting should always
include two of three days of juice fasting before and after
the water fast. This alternating between juice and water
fasting is the most effective method of achieving a full
cleansing through fasting.

--------------------------------------------

but as a rule of thumb you are suppose to drink half your weight in ounces per day. doctors will tell you that it is only safe to fast for 3 days unsupervised on a water fast. people on here have gone longer.you should research on signs that point to you needing to break the fast.
good luck!

How much water you should drink a day depends upon your body weight and the time you spend for exercise everyday.

In a general rule, if you are 140 pound weight, you should drink 70 ounce water every day. Plus, if you are exercising everyday, add more water to your diet.

To lose weight, you have to drink right amount of water on right time.

Drinking water before each meal will supress your hunger and you will eat less. And adding 500 ml of water to your daily diet can help you lose 10-15 pound body weight.