I vary my frequency to the customers requirements, generally 4 weekly, some 8 weekly, some 12 weekly.
I have one account that is only ANNUAL

But I don't mind, I can live with £120 for a mornings work

Again, generally I stick to 4 weekly, if customers want it more often than 4 weekly it depends on their location and whether I have to go out of my way to clean them.
The same with any cleans from 4 weekly up to 8 weekly. Any multiple of 4 weeks is fine by me.
I try not to get caught out with one off's and I charge extra if a customer justs wants a one off clean.
A true initial clean is differrent, if there is putty, paint and other crud that means you are having to knife off every pane of glass, or spend double the amount of time cleaning them, then the customer is told they will have to pay accordingly.
Actually I have few domestic accounts now............err, well I did only have a few, since going over to WFP a few months ago that is changing quite rapidly

Most off my work is commercial now, but the above has always applied to domestic accounts for me.
Ian