The right frequency depends more on your household than a single fixed rule.
General guideline
Most homes benefit from a professional carpet clean every 6 to 12 months as general maintenance, even without any visible staining.
Households with pets or young children
Every 4 to 6 months is more realistic, since higher foot traffic and accidents accumulate faster than in a typical household.
Allergy-prone households
More frequent cleaning helps manage dust mite and allergen build-up, alongside regular vacuuming between professional cleans.