During a brew day, you can expect to use about 115 gallons of water. Please keep in mind this can change some depending on the recipe, your process, etc.
Here is a breakdown:
1) For a standard 1 barrel batch using 80 lbs. of grain, you would use roughly 50 gallons of water for the beer itself.
2) Steam condenser mister. This uses 25 gallons per hour so about 25 gallons for a brew day. This can be reclaimed and used to clean as it will be hot.
3) Chilling wort. This uses about 50 gallons to chill 35 gallons of wort. This will also be hot and can be saved for cleaning if needed.
4) When cleaning the tanks a hose may be used to spray things down, which usually equates to ~5 gallons