How can you charge double for only cleaning them every 8 weekly, people aren't stupid
I wouldn't pay £20 to get my windows cleaned every 8 weeks if I could get them done every 4 weeks for £10 each clean.
Another window cleaner will come along and seriously undercut you and offer the customer a much better service.
I've marked 4 weeks as the majority of ours are that, though we also have weeklys and fortnightlys (all commercial), 8 weeks, 12 weeks, 6 monthlys, and "on requests".