I live in a flat 14 floors up. I use the tap in a the car park which has meant I've learnt to be carefull and intelligent about how much water I use when working because although I use a merlin (plus di) it still takes time every evening to fill up.
I reckon on about 20 - 25 ltrs for a large deteched house plus conservatory. I may not do it as quick with those that can have the flow on full whack but as neccessity is the mother of invention, I've learnt what works best and most efficiently when pure water is at a premium.
For a the standard CIU 3 bedroom semi, I use maybe 15 ltrs.
For first cleans, I use the customers tap water first to get the dirt off - with their permission of course. I use a 25ltr barrel that I fill from their tap and as I use a DIY trolley system as both a van mount and as a trolley, I can pick and choose the best way to do the job.
If they have no outside tap, then I just have to use the pure stuff but even then, I'll wet the glass and frames, turn the flow off, scrub, maybe give it a quick rinse, turn the flow off again, scrub, and then wash and rinse as normal.
In a day I have never used more than 200 ltrs and generally I use 150 - 175 ltrs and sometimes even less than that.
Added later: It's best to just get out there and give it a go and then re read the advice given and other posts on similar subjects because then it has more meaning to your situation.