I wouldn't like this idea myself because if i got people to agree to a price per week they would probably expect their windows to be at least checked if not cleaned on a weekly basis.
It would be great if you could turn up 4/6/8 weekly but the reality is that a bird will sh!te on a window within a day or 2 of cleaning.
I don't see, unless you checked windows on a weekly basis that you could guarantee clean windows for the year.
And if you did check on a weekly basis your fuel expenses would be several times more than doing once a month.
I can only see an administration headache with high fuel costs if the customer was of the mindset of paying weekly (for a weekly service if necessary).
The basic idea is that on a price per week basis the customer agrees to a set weekly figure, if they are paying per clean and feel that their windows dont need doing as often they may feel that they are being "ripped off".
But if they pay per week then you dont need to visit as often, gain a better rate per visit and can take more time when you do visit to justify the price you charge.
Its not an invitation to call round everyday to make sure a bird's not sh*t on the windows. Obviously the customer is made aware of this from the start.
"The method we use to clean your windows means they stay cleaner for longer, so we can save you money by not having to call as often. Using the old method of cleaning would cost you £3 a week but this way it will only cost you £2. etc etc"
Mark