Depends on the length of the duty, how thirsty i am and so on. Usually around 1 to 2 litres. However, the most i ever did was 6 litres in very warm weather without an APU after having cycled to work. Not normal though, and these days i plan a bit more carefully as the most i get for free is 2 litres a day, except if someone else from the crew doesn't drink their 2 litre allowance...