Exterior paint needs two things to cure properly: a dry surface and temperatures that stay above roughly 50°F through the night. In the Seattle area that reliably lines up from about mid-May through September. Push into October and you're gambling on a dry stretch; try it in March and the siding is still holding winter moisture even when the sky looks clear.
The prep work is what really eats the calendar. A full exterior means pressure washing, then a day or more of drying, then scraping, sanding, caulking, and priming before the first finish coat goes on. Rain in the middle of that sequence doesn't ruin the job, but it does stretch it, which is why we build weather days into every exterior schedule instead of promising a date we can't keep.
If your house is showing the early signs — chalky residue on your hand when you rub the siding, hairline cracks in the caulk around windows, paint lifting at the bottom edge of boards — get the estimate done in spring so you're on the summer calendar. The best exterior slots in June and July book up early.
One more thing worth knowing: the direction your walls face matters. South and west exposures take the sun and fail first; north walls stay damp and grow moss. A good exterior painter treats each side according to what it's dealing with rather than spraying the whole house the same way.