i reckon it depends on what part of the country you live in etc in regards to earnings as a one man band domestic window cleaner.
i think if your down south then £4000 a month would be a good round.more like £3000 a month further north.
dont forget you wont work 20 days a month due to illness,holidays,really bad weather etc.plus when you take off tax,insurances and expenses itll be less than that but still better than slogging your guts out 9am-5pm for someone else!!
as for how many customers well it just depends on so many factors but i have 340+ mostly monthly and i can get round easily and still have a few days off here and there during the month.also i finish around 330pm-4pm most days.starting at 9am-10am.my work is a mixture of very compact estate work,large domestic and a few offices and other commercial jobs.
i personally dont want to be working flat out week in/week out every week so i dont!!
the hard part is getting the work and organising it to make it pay.
you can earn more than the figures ive quoted above but it just depends on your work/life balance and how much you need/want to earn.
and of course pricing!!