What drives the interval
Four variables do nearly all the work. Volume burned is the obvious one. Fuel moisture is the biggest single lever, because energy spent boiling water out of wet wood is energy not spent carrying smoke up a warm flue. Operating temperature is the third: a stove damped hard for a long overnight burn deposits far more than the same stove run hot. And flue temperature is the fourth, so an exterior chimney on a cold wall and an oversized flue both accelerate accumulation regardless of how well you burn.
Why an annual inspection is separate from sweeping
Sweeping removes deposit. Inspection assesses condition. They are commonly done in one visit because a clean flue is far easier to examine, but they answer different questions, and a chimney that needs no sweeping this year can still have developed a defect. The inspection is what catches a cracked tile, a deteriorating crown, a failed connector or a clearance problem before it becomes the reason for an incident.
How to find your own interval
Have the flue inspected at the end of a normal season and ask what a season produced. That single measurement accounts for your wood, your appliance and your habits all at once, which no general rule can. From there the interval is a straightforward matter of arithmetic, and it can be revisited whenever the wood supply or the way you burn changes. Redo it after any change.
The signs that say do not wait
Bring it forward if the glass is blackening much faster than it used to, if the appliance has become harder to light or slower to come to temperature, if you can see hard shiny deposit in the flue, or if smoke has started entering the room. Those all point at the same underlying conditions, and they are worth acting on, not logging.