My round use to be 90% 6 weekly, 10% 8 weekly.
Iv recently converted it to 90% 8 weekly, 10% 6 weekly mainly because after 8 weeks, no matter where you live your windows need cleaning. The only time I ever get a 'leave it is this time' now is if the customer is having building work etc.
8 weekly customers are much happier to see you IMO.
I also put the price up by an average of £2 for every house, some a bit more, to cover for the extra dirt.
A good way of explaining this to the customer was showing them that even though the price per clean was going up, the yearly price was considerably less due to the fact there was a bigger gap between each clean.
It was a big gamble on my part implementing so many changes in one go and I was prepared to lose a fair few customers. I lost one!
What these changes meant for me overall was that I had to do considerably less work each week to earn the same amount of money, and I freed up an entire day a week to either pick up better priced work or to leave free for extra jobs, con roofs, gutters etc.