There is no single correct interval. A quiet two-person household with hard-floor hallways is a different proposition from a family home with a dog and a garden gate that never closes.
The useful question is not 'how often' but 'how quickly does this carpet reach the point where soil is doing damage'.
What shortens the interval
Each of these factors increases soil load or the visibility of soiling.
- Pets, particularly in households with garden access
- Children and regular floor play
- Light-coloured or plain carpet, which shows traffic lanes early
- Entrance halls, stairs and landings
- Smokers, open fires or cooking odours reaching carpeted areas
- Allergy or asthma sufferers in the household
Why waiting costs more
Dry soil is abrasive. Every step grinds particles against fibre, cutting and dulling it. That damage is permanent — cleaning can remove the soil but it cannot restore fibre that has been abraded flat.
This is why planned cleaning is a maintenance cost rather than a cosmetic one, and why carpet that is only cleaned when it looks bad rarely recovers fully.
Between professional visits
Vacuum high-traffic routes more often than the rest of the floor, use barrier matting at entrances, and deal with spills immediately by blotting rather than rubbing.
Check any manufacturer warranty on newer carpet — some specify periodic professional cleaning and require proof of it.