if you dont charge more for 4/6/8 weekly cleans you are leaving money on the table and you are costing yourself more time.
I will point out a few things for the people that say it does not take longer:
if you do it less often you:
You need to use more water, marginaly more not much but more scrubbing, there is possibility of twice the bird muck (you have to allow for this or it cost you time, time is money
You will have to do 2 houses instead of one to make the same money, this without doubt takes more time traveling, (collecting if you do that) wear and tear on the van, equipment, clothes and your shoes

admin time as there are double the amount of people to account for.
You will also have to pay double the cheque charges if you run it as a business and not a personal account.
there is a list I could sit typing about why doing it less often costs more money, good for you if you dont charge extra but now you cannot say it doesnt take any more time as I have just shown you only a few things that we all need to do.