Edberg is correct that from the NH standpoint, you should drink only to thirst. However, there are a number of modern and not so modern Hygienists who feel that we're not always in tune with our thirst (or our hunger) sense and should ensure that a minimum of water is consumed.
Albert Mosséri insists on at least 1 litre to be consumed between midnight and midday when the elimination is strongest and if this is difficult, he suggests adding lemon to the water. (The Fast - Nature's Best Remedy http://rawfoodconsultations.com/book.html)
Loren Lochman of Tanglewood Wellness Center, Panama, also feels strongly about drinking regularly during the fast.
Perhaps the best thing is to monitor your intake and if thirst is not inspiring you to drink about 8 glasses per day, then just meditate on it and see what answer comes to you.