When I first started I offered a choice of 1 or 2 monthly.
I carried on offering that until my round became overfull when I changed to 6 weekly.  The few two-monthly customers I had were okay with that, and the monthlies seemed to not even notice.  It was 6-weekly or nothing for quite a while.
Then I started to get requests for less frequent work.  At first, I would only do this for the much larger jobs, but made sure that I charged well for them.  I ended up turning away a fair bit of smaller work that wanted 12-weekly cleans.
In the end I relented, but started charging 50% more for them.  This eventually settled down to charging 1/3 more, rounded up to the next pound.  That's pretty much where I am today, but I had a couple of larger 6-weekly  jobs that reduced frequency, but only tacked a fiver on because I really wanted to keep them (they would still have been excellent prices with no increase).
I do make it clear to everyone that 6-weekly may occasionally take a couple of months through a harsh mid-winter or if I'm on holiday or sick, and similar to the less frequent customers but say maybe around four months.  So long as the frequencies are a multiple of each other, it means there's no need to drive around all over the place.