For me a standard 3 semi will use a minimum of 15 litres.
you may well use less on occasion, but on other occasions you will use more.
Ergo; a good rule of thumb is 15l per average semi.
With continuous usage throughout the day, doing house after house, doing all of the windows, you are going to need closer to 300 litres minimum. That'll do you approx 20 houses.
Not all houses are identical of course, some will be smaller, some will be larger, but you get the idea.
I carry eight 25l containers with me, and that mostly lasts me the whole day, but a couple of times a week I'll have to go back to top up. A good excuse for a coffee and some toast
Regards,
Ian