Many years ago we used to clean weekly. Then we started getting the same thing 'leave it this week' so the general trend went to fortnightly, with the thinking being they wouldn't be so ready to skip a clean. That worked for a few years then it started again, so we all started doing it monthly, and told our customers that as we were now leaving it as long as we could between cleans, if they wouldn't have it done we would cancel them.
Now we always try to get new customers signed up to monthly cleans, but if they aren't happy we offer every other month instead.
I can see the time coming when no-one will want it done monthly, so bi-monthly will become the norm.
Keep this up for a few more years and we'll be doing it the American way: once a year