I have had a few go 8 weekly but thats more down to money.
No-one has said to stagger the work but if they did I would say that rather than clean your windows, let them get dirty, then me clean them again - I say that with continual cleaning you can enjoy clean windows all year round.
If I was to stop the process would have to start again re: first cleans.
So really, they are better off? Rather than have 3 weeks of clean windows then 1 of dirty they have 4 weeks of clean windows.