South Dakota puts an exterior through an unusual amount: freeze-thaw cycling all winter, wind-driven rain and hail, and full summer sun with very little humidity. Every one of those attacks the coating differently. Freeze-thaw works water into any gap and levers the film off, and UV breaks the binder down from the surface.
Prep is the single biggest factor in how long a coat lasts, and it is invisible once the job is done. Paint applied over chalky, dirty or damp siding is failing from the day it goes on, no matter how good the product is. Paint over properly cleaned, scraped and primed wood can hold for a decade.
Exposure varies enormously on the same house. South and west walls take the most sun and typically fail first, while a sheltered north wall can look fine years longer. It is normal for one elevation to need attention well before the rest.
Substrate matters: wood siding needs recoating sooner than fiber cement or vinyl, and previously-failing paint tends to keep failing at the same spots, because the cause (usually moisture getting behind it) is still there.
The useful signal is not age, it is behaviour: chalking that comes off on your hand, hairline cracking, paint lifting at the bottom edge of boards, or bare wood showing at the south-west corner. Those mean the film has stopped protecting the wood. At that point waiting costs more, because bare wood weathers and then needs repair before it can be painted.