During a brew day, you can expect to have ~75 gallons of water that will end up down the drain.
Here is a breakdown:
1) 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.
2) 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.
3) When cleaning the tanks a hose may be used to spray things down, which usually equates to ~5 gallons