Condenser size/effectiveness/whatever ends up being the limiting factor. The Jay Leno video someone linked has him stating that the car gets 200 miles out of the water in warmer weather (which isn't a huge problem, water is cheap, but radiators are pretty well advanced technology at this point so there isn't much reason to expect big improvements).

Someone at NASA analyzed it once:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/1970001...