Well, on Saturday's ride it took me about 7.5 hours to go through 3 liters of water. That would imply (if water consumption is proportional to time) that 2 liters would last for 5 hours, which is actually pretty far especially if you're going faster than ducklings-bike speeds, and you can probably find more water somewhere along the way (maybe in the restroom you'll need during that time too).