In Birmingham, I’ve noticed windows can go from “fine” to “obviously grubby” faster than you’d expect, mostly because of traffic film, rain spotting, and general city dust. For customer-facing businesses, I usually recommend at least monthly cleaning as a baseline, and often fortnightly if your site is near a main road or you have lots of glass at street level. I once worked in an office where the lobby windows were the first thing visitors saw, and we learned that even small smears looked worse when the sun hit them mid-morning. We moved from monthly to every two weeks, and the difference in first impressions was bigger than I anticipated.
A practical way I decide frequency is by splitting windows into “zones.” Zone one is the entrance and street-facing glass where people queue, browse, or wait. Zone two is upper floors or less visible sides. You can clean zone one more frequently and do zone two less often, which keeps standards high without overspending. Also, consider seasonal shifts: spring and summer can bring more pollen and dust; autumn brings wetter weather and more splashback near ground level.
While looking into this topic I found Art Cleaning to be useful.
On a related note, if your frames and sills are neglected, even perfectly cleaned glass can still look tired, so I always ask for those to be included periodically.
