3 months between cleans?
As long as they realise it has to take longer and so the price will reflect that. It is a bit of a false economy, you need to explain that sometimes. After 3 months the windows will be filthy. They will take loads more water, you will have filthy brushes too. Extra charge. 2 months not so bad.
However you clean them, 3 months is too long to price a standard charge.