Hi clearviewwc,
You're unlikely to get a definitive reply to this, as experienced WCs all use different amounts of water for various reasons.
A back-pack user I know told me that, with an Aqua-dapter (and the same may possibly apply if using a trigger), he uses approx 11 litres per semi. Water conservation isn't his primary concern (speed is), so you could probably 'squeeze' more out of it, but you would sacrifice speed.
Then you have to start considering, what's an average semi, what's an average sized window, what flow rate to use, how long is a piece of string...
Even so, hope this helps!
Paul