Not neccessarily the case Richy. Just because they pay a monthly charge, which in all likelihood they do, that the windows are meant to be cleaned on a monthly basis. The amount they charge is dependant on what the running costs of an apartment of flats is. Theres allsorts of communal charges, lighting, gardening of communal areas, carpet cleaning/replacement, guttering, roofing, communal paths, fencing. Allsorts.
If we used your analogy Richy they'd be getting their carpets cleaned monthly and new fences every month. Which doesnt happen, a four yearly window cleaning visit will simply be broken down into a 12 monthly budget.