the common recommendation is 8 cups/day, but it really depends on what you're doing. if you live in a hot climate and sweat a lot, then you'll have to drink more, if you work out a lot, you'll have to drink more. during the summer i'll usually drink over 4L of water because i usually work outside and i row twice a day. just watch your urine, if it's a dark yellow colour, you should drink more, if it's clear or almost clear, you're drinking enough.
another thing to remember is that you don't just sweat water, so if you're doing lots of exercise or are working outside and sweating a lot you should be replenishing the electrolytes your body's losing. pick yourself up a good sports drink so you don't cramp up. I like this stuff called eload, it's designed with athletes in mind, it's just a powder that contains salts, natural flavour, no artificial colouring, and dextrose as opposed to fructose syrup, which is easily digested so you won't get the kind of stomach cramps gatorade can give you.