It's an unanswerable question. Everyone's different, so some will use lots and some a little, and still achieve the same result. I believe that our 400lt tanks are quite enough for a day's work, and none of the blokes ever complain of running out.
One tip to save on first cleans: spray the glass, turn the water off, scrub like billy-o then turn the water on again and rinse. Repeat until clean. If you have to scrub over and over to shift stubborn muck, the water that's running from your brush is wasted - provided you wet well to start the scrubbing will be just as effective as if you have a constant flow of water.