if you dont do the same area when you do this other custy that want to change to every 12 weeks, then i dont see it a problem, id do it.
However if i was already in the area doing others at 10 weeks i wouldn't change to 12 weeks just for 1 customer! if they wanted to change to 12 weeks id have to increase by £2 say just for the extra travel just for that job.
I would reply saying it wasn't viable as a reliable business you provide to all your customers to do it outside of the times you do it already, as you are in the area doing other cleans at the same time. If you have to visit at different times, then you couldn't offer the same level of customer service to your other customers as you have to keep a reliable service to all your customers. If they wanted a bespoke service then there would have to be an extra charge for it for the extra travelling and time out taken from other areas of your rounds/service
You may lose that customer but im sure the loss would be offset the time out you have to take to do them from your other areas and travelling costs etc to get to them.
Who's running your reliable business you or your customers. Keep control advise you cant do it, without being reliable to the rest of your clients elsewhere. Unless you want to be running everywhere at all times.