The twice-a-year baseline
Spring and fall works for most homes. Spring clears winter's accumulation and the worst of the pollen season; fall gets the glass clean before the shorter, darker days when you want maximum light coming in.
What makes glass get dirty faster
Irrigation overspray is the big one and the one people most often don't connect. Sprinklers hitting ground-floor windows leave mineral deposits every single cycle. Over a season those build into hard water spotting that ordinary glass cleaner won't remove — and over years they can etch into the glass permanently. Adjusting the sprinkler heads is genuinely worth doing.
Tree cover means sap, pollen and organic debris. Spring pollen in particular bonds into a film that a rinse won't take off.
Proximity to busy roads means road film and exhaust particulate. Near the coast or the bay, salt air adds its own deposits.
Construction anywhere nearby coats everything in fine dust that's abrasive — worth cleaning off sooner rather than letting it sit.
Screens and tracks are half the job
This is the part most people skip and then wonder why the windows looked dirty again within a week. Screens hold dust that transfers straight onto clean glass the first time it's windy. Tracks collect grit, and grit plus water equals a dirty streak down the glass.
Cleaning glass without doing screens and tracks is doing a third of the job.
Hard water spots and what's realistic
Light and moderate mineral spotting usually comes off with the right treatment. Deposits that have been building for years may only improve substantially rather than disappear, because the minerals have begun etching into the glass surface itself.
Any company that promises complete removal of severe long-term hard water etching without looking at it first is overpromising. The honest answer is that it depends on how far it's gone.