Im in Oxford UK and I find 6 weekly is best to offer as most customers prefer this frequency. However, I am from Swords in Dublin originally and my Parents road get cleaned 'whenever' the window cleaner (polish guy) gets around, usually every 3ish months.
I think even the poles have adapted to the easy way of Irish life

My advice though is to set a frequency and stick to it. When I started 3 years ago I offered 4, 6 or 8 weekly to each customer but when my books got full it was a nightmare as some weeks are over full and some are virtually empty.
Ive now changed most to 6 weekly and its much better as it is more evenly distributed over the weeks
Adie