I've offered longer cleaning frequencies for a few years now, and they're all my best work; mostly for larger type properties (which I class at £20 plus).
You charge high and soften the blow with a longer cleaning frequency. Tell the customer this and point out that not all window cleaners offer this service (they don't). Sometimes the customer may first appear to want them cleaned more regular, but even here I'll explain that many customers start out that way, then once the novelty wears off, they're not so keen to have a short-frequency-clean, and that a slightly higher charge and longer frequency tends to work best for my customers.
If you charge a good price and turn up every-four-weeks they'll groan, but if you charge a good priced and turn up every 8 to 10 weeks, they're less likely to moan/drop you.
The only downside is it takes longer to build up this clientèle.
That's been my experience.