If your customer base is happy with fortnightly then stick with it!!!!
You need half the customers for one thing!
And your life will be so much easier and quicker.
Around our area, fortnightly isn't heard of, it is all monthly or 2 monthly (for domestics of course)
But don't mix things up by having odd numbers!
such as 3 weekly or 5 weekly, stick with 2 weekly, 4weekly, 6 weekly and so on.
Some work stictly on a monthly basis as against a 4 weekly basis, i hate this system personally, and much prefer to base things on weekly rather than monthly.
But whatever your system, don't let customers throw you a loop ball by asking you to clean their house outside of your normal rota, any multiple of your normal rota is fine.
So if its mostly a fortnighly clean, any multiple of 2 weeks is fine for you.
I wish I could persuade all my domestic accounts to be fortnightly!!
Ian