Seattle, WA
For most Seattle-area homes, the baseline is twice a year -- once in spring after peak pollen season, once in fall before the heavy rain sets in. Homes with significant tree cover often need quarterly cleanings to stay ahead of sap and debris.
Spring cleaning clears the pollen that built up over the previous months, and a fall cleaning gets ahead of the rainy season before dirt has months to bake onto the glass.
Homes under large conifers or near flowering trees accumulate sap and pollen faster than open lots, which is why quarterly service makes sense for some properties and not others.