Short answer: every 12 to 18 months for an average household. Every 6 to 12 months with pets, small children, or anyone with allergies.
By household
| Household | Frequency |
|---|---|
| One or two adults, shoes off | Every 18 months |
| Family, no pets | Every 12 months |
| Family with pets | Every 6–9 months |
| Allergies or asthma present | Every 6 months |
| Rental between tenancies | Every tenancy, if required |
Why manufacturers care
Most carpet warranties require professional cleaning at a stated interval, commonly every 12 to 18 months, and require you to keep the receipts. Skip it and a warranty claim for premature wear can be declined.
The reasoning is the same as for vacuuming: grit at the base of the pile cuts fibres underfoot. Domestic vacuums lift surface debris well and deep grit poorly. Hot water extraction is what actually gets it out, and doing that periodically is genuinely what keeps carpet alive rather than an upsell.
The methods, briefly
Hot water extraction (often called steam cleaning). Hot solution injected under pressure and immediately extracted. The most thorough method and the one most manufacturers specify. Drying takes 4–8 hours with good airflow.
Encapsulation. A polymer that crystallises around soil, then gets vacuumed out. Faster and dries in about an hour, but less deep. Good for commercial maintenance between extractions, see commercial carpet cleaning frequency.
Dry compound. Absorbent compound worked in and vacuumed out. Minimal drying time, least deep. Useful where a floor cannot be out of use.
Bonnet cleaning. A pad on a rotary machine. Cleans the top of the pile only. Fine for appearance between proper cleans, not a substitute for one.
For a home, hot water extraction is almost always the right answer.
What it costs
$40–$70 per room in the Waikato, with a two- or three-room minimum common. A three-bedroom house with lounge and hallway usually lands at $160–$280. Stain treatment, deodorising and protector are typically extra, ask which are included before comparing quotes.
Getting the most out of it
Vacuum thoroughly first; most operators will, but a slow pass beforehand helps. Point out specific stains and say what caused them, because treatment differs entirely between wine, pet accidents and oil. Keep foot traffic off until fully dry, and open windows or run a fan.
Do not over-wet, and be wary of anyone who does. Carpet that stays damp for more than about 24 hours risks odour and, in a New Zealand winter, mould in the underlay.
For the tenancy-specific question of whether you are actually required to do this, see do you need professional carpet cleaning at end of tenancy.