A question that has to be analyzed based on area of your home, depending on daily public traffic(cars,busses,trains ) and also the amount of surrounding trees
First of all, think about the amount of rain in your area, all sources of pollution (both natural and human-caused).
Secondly, just observe how fast your windows attract dirt. You might take a notice on how often people on your street wash their windows.
Thirdly, re-consider your cleaning solutions. Nowadays, it is easy to fall in love with all those beautiful packagings and promises to gift you with the most beautiful sparkling windows. But the truth is that you don’t need all those chemicals (for the cases of mediocre pollution levels in the area).
For highly polluted areas, it might be once or even twice per week. For windows in the most households, the golden ratio might be once or twice a month, depending on how close is the sea, forest, sources of sand, industrial dust and other dirt factors. If you live in a perfectly balanced environment, you windows might enjoy bi-monthly cleaning. The point here is not to leave them on their own for more than 3 months.