Hello M'dear,
Cambridge state that you should drink a minimum of 4 pints of fluids a day which is equivalent to 2.25 litres.
Many people advocate drinking more than the minimum recommended amount as they find it is beneficial to their weight loss - see Icemooses thread above on the forum sticky threads list. I personally drink between 3-4 litres of water/herbal teas a day.
If you drink too much water over a short period of time this can be dangerous to your health - the amount of water you can drink safely differs from person to person and is dependent on a variety of factors including height, weight and level of exercise etc - google brings up some informative articles on the subject. I find that my max limit on a normal day with no major excercise is about 5.5-6 litres (I am 5ft 7 and c 10st 9lb) but everyones optimum and maximum water intake will vary.
If you stick to Cambridge's 4 pints minimum that is a good place to start and you can increase your intake from there if you wish.