The answer may depend on many factors – not least, on whom you ask. Dr John Kazer for example, who is a Carbon Footprint Certification Manager for the UK-based Carbon Trust will tell you that for a 250ml cup, it’ll take around 30 litres – but that’s just for the tea itself – considerably more if you add milk and sugar :
“Somewhere around 30 litres of water is required for tea itself, 10 litres for a small dash of milk and a further 6 litres for each teaspoon of sugar. This means that a simple cup of tea with milk and two sugars could actually require 52 litres of water – enough to fill my kettle more than 30 times.”
More info, see: The water footprint of food from Professor Arjen Y. Hoekstra, Twente Water Centre, University of Twente, the Netherlands.