Duct cleaning frequency depends on a few factors beyond just time elapsed, though there's a reasonable general guideline to start from.
The general recommendation
Most homes benefit from a professional duct clean every 2 to 3 years as a baseline, which keeps dust accumulation from building up to the point where it's noticeable every time the system runs.
Factors that shorten the ideal interval
Homes with pets, renovation work, nearby construction dust, or allergy-prone residents may benefit from cleaning closer to the 2-year mark rather than stretching to 3, given these factors all increase how quickly dust and allergens accumulate.
New homes and systems
Even in newer homes, construction dust from the build itself often settles into ductwork before a system's first real use, which is why some newer properties benefit from an initial clean earlier than the standard 2 to 3 year guideline might suggest.
Signs it's time regardless of the calendar
A dusty smell on first use each season, visible dust around vents, or worsening allergy symptoms during heating use are all reasons to book a clean regardless of exactly how long it's been since the last one.
Building it into seasonal maintenance
Scheduling duct cleaning in early autumn, ahead of the first cold snap, is a practical way to keep it on a regular cycle without needing to track it separately from other seasonal home maintenance.