I'm currently following the advice of a both a kidney consultant and a GP, who agree that 3 to 4 litres per day is the target to aim for. (Not just for me, who ended up in hospital with kidney problems earlier this year, but for everyone).
So I start my day with a pint of squash followed by two mugs of tea and then I monitor the colour of my urine throughout the day to check that I'm not falling below my target of 7 pints (4 litres) of fluid.