I think there are two things to think about when deciding how much water to drink: what's the minimum you need, and how much is too much.
The 8 glasses per day rule comes from old US medical advice and equals about 1.8L of water. Many medical experts consider this advice outdated. Whether you need that much depends on how much you are sweating, and how much water you get in the food you eat (we actually get quite a bit of water this way). Some health authorities recommend as little as 1.2L of water per day in colder climates, others recommend up to 2L. What they all advise is letting your thirst tell you how much you need to drink. If you are thirsty, drink water. If you are hungry between meals, drink water. Thirst sometimes masquerades as hunger so sometimes a glass of water will fix it. However often hunger is simply hunger so if you're still hungry you know you need to eat something!
The other thing to consider is whether you can get too much of a good thing. Firstly, a study done last year discovered there is very little scientific evidence that consuming extra water on top of our daily requirements has any health benefits. People have claimed it's good for weight loss, skin, kidney function and headaches but there appears to be virtually no evidence from published scientific research to back this up. Secondly, too much water can be dangerous. There have been instances where excess water consumption has caused brain swelling and death. Here we're talking about 4L in two hours, so not really something you could easily do accidentally, but it's something to bear in mind when you force yourself to drink a lot of water.'Just add water', Journal of the American Society of Nephrology (opens a PDF)BBC: Lots of water 'is little benefit'.BBC Why is too much water dangerous?
Now if you excuse me, my mug is empty so I'm off to get myself another glass of water.