when you first start you will be using lots of water so you
will be lucky to get one house per fill,but as you get used to it
your water usage will come and you will do more for each
fill.
i can do two standard semis and at least three standard terraced houses
with 18 litres of water!!!!
no sorry have to correct myself there,i meant i 25ltr barrel
of water 
you dont have to blast water at windows to clean and im
quick
i use a 15 ltr backpack
its not "BLASTING" water i use a 60psi surflo with a mk3 analogue varistream set at about 70percent with a gate valve to turn flow on/off using 100mtrs of microbore. Pressure and flow are different things
i wasnt particurlary aiming the comment at you,maybe you
might consider you may and im only saying may be using more
water than needed
standard terrace round here generally has 6 windows plus
i patio window and front door and i can definatley do 3 and
bit of these houses on maintenance cleans with one barrel of
water
i use a ws15 with a 70psi pump which chucks out 2.6ltr per min
by time gets tobrush it prob a little less im really quick and on
my 4 weekly cleans i rarely have to rinse unless there is bird
crap or something stubborn hanging on,as long as im using
000ppm i never have spotting issues