A "standard" cup of tea is 2.5 deciliters, and that is, of course, the amount of water you need to brew a cup. But how much does it cost to produce the leaves and get them to you? On average, a total of 27 liters of fresh water is used. Liter! Most of the water needed is not what is in your cup, but what is used earlier in the chain.
All this, together with examples for different products, can be read about at Water Footprint Networks website.
What's fun to read in the context: While a cup of tea needs 27 liters of water, a cup of coffee (you know that funny herbal tea made from beans...) needs a whopping 130 liters of water. So at least in terms of total water consumption, tea is superior. So the fine print: To be completely fair, you have to look at the amount of water used. Sometimes you need completely clean water, sometimes you manage with rainwater, sometimes with so-called "grey" water, which you can perhaps give to plants, but which you don't want to drink.