I dont think it has to depend on turnover. Many Window Cleaners have a volume of work but are more concerned with having the time for other pursuits, so take on people to cover the excess work. This has been the case for me in the past when I only wanted to work two and a half days.
From a purely bussines perspective though, to take on a full time employee a turnover of £50000 ish upwards.
You are doing well if you get anywhere near 50 000 alone, its a tall order, im sure one man could clean that amount with a bit of hard graft but the struggle is getting there, if you are at 30 000 its hard to clean , canvass, and do new cleans and keep our schedule while you get to 50k
I am employing part time to take the load off while i canvass harder, and do first cleans then when at full capaity again take on....
Good luck though if you get 50 000, im sure some do, but most will find that hard on mainly residential