most of my regular work is roughly calendar monthly/2 monthly.it allows for greater flexibility esp in winter with bad weather when i can be a few days or a week or so behind sometimes.
i do have some stand alone work that is 3 monthly and the odd 6 monthly but these are very few.
i find most customers are happy with a monthly service and as for windows still being clean im finding that they are not in the vast majority of cases!due to buildings being near trees,main roads etc.
dazmond