It really depends a lot on your system.
Extract, partial boil, and chilling in a water bath: Probably a lot less than that.
All grain, full mash and boil, with an IC chiller: It'll be more but once again that depends on your system (how much you're losing to dead space, grain absorption, boil off rate, etc). If you're using an immersion chiller and running the water off and not recirculating/reusing it somehow you could potentially blow through a lot of water.