Hi Seamus,
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 around 11 litres per semi with approx 10 windows (4 front, including door - 4 back - 2 on the side). Water conservation isn't his primary concern (speed is), so you could '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. As Bob said, how long is a piece of string...
Hope this helps.
Paul