Nearly all my work is 8 weekly.
Your right mick, windows are normally pretty dirty by the time 8 weeks comes around. It means I almost never get a 'not this time', nor do I feel any resentment about me being there from my customers.
The other great thing about cleaning dirty windows is that you can noticeably see the difference once they've been cleaned, and customers appreciate your work more.
With wfp 4 weekly cleaning is not needed in most areas, I noticed this almost 6 months after switching to wfp, windows still looked clean after 4 weeks. With that comes the 'not this time' and 'the windows are still clean' comments.
6 weekly is probably just right in terms of frequency but I wanted to enforce a considerable price hike on my customers, so switching to 8 weekly softened the blow.
I only lost 1 customer after the big price hike and switching to 8 weekly, my earnings stayed the same but I had much easier days ( only do 6-10 houses a day) and plenty of space in my rounds for the add ons like gutters and cons.
Works for me and customers seem happy..