The annual baseline
For a typical Peninsula home on an open lot with moderate sun exposure, once a year keeps siding ahead of organic growth. That timing exists because algae and mildew grow slowly enough that a year of accumulation still cleans up easily, but two or three years of it starts to set in and stain.
What makes a home need it more often
Tree canopy is the single biggest factor. Homes under mature oaks or redwoods — most of Atherton, Woodside, Portola Valley, Los Altos Hills — get more organic debris, more shade, and more sustained moisture. All three feed growth.
Orientation matters nearly as much. North-facing walls get the least sun and stay damp longest, which is why one elevation of a house often looks noticeably worse than the others.
Proximity to the coast and the fog belt is the third factor. Daly City, South San Francisco, San Bruno and the western neighborhoods of San Francisco stay damp far more of the year than inland cities, and organic growth is close to a year-round condition there rather than a seasonal one.
Irrigation is the one people forget. Sprinklers that hit the house repeatedly keep a strip of siding permanently damp and often leave mineral deposits on top of it.
What happens if you wait too long
Growth that sits for several years does two things. It becomes harder to remove, sometimes requiring longer dwell times and repeat application. And on porous surfaces like stucco it can leave staining behind even after the growth itself is killed and rinsed away.
On roofs the stakes are higher — moss holds moisture against roofing material and works under shingle edges, which shortens the life of the most expensive surface on the house.
How to tell it's time without guessing
Look at the north side of the house and the wall sections under the eaves. That's where it shows up first. A green or gray tint, dark streaking below the roofline, or a chalky feel when you run a hand across the siding all mean it's time. If you can see it clearly from the street, you're overdue rather than early.