if your doing less often then there should be an extra charge, simple fact it will take more time.
I give the customers what they want, if they want 4 weekly they can have it if they want 8 they can they they want less often they can have that too, I did think of six weekly but why upset the apple cart, many do this because they are lazy they could be cleaning 4 weekly then it drifts to 5 then 6 (they just would'nt admit it on here (in my book that loses money and thats why I am in business) it loses a hell of a lot more than the not today brigade, if you say your tern clearly when starting out the not today dont happen very often at all what ever the time interval.
This of course is IMO but also from what I do daily, I dont have half the problems many seem to have on here.