The honest answer most carpet manufacturers actually print on the warranty: every 12 to 18 months. Homes with pets, kids, or allergies should think 6–12. Heavy-use commercial spaces should think quarterly. Vacuuming, while useful, doesn't extract the deep contamination that builds inside the carpet pad and backing.
What's at stake when you push past those windows is two-fold: indoor air quality drops as embedded allergens and bacteria accumulate, and the carpet wears prematurely because trapped grit literally cuts the fibers every time you walk on it. Replacing the carpet costs 10–25× a professional clean.
Spiker's hot water extraction process is designed to meet the manufacturer-recommended cleaning standards. That's the line we won't cross, because crossing it puts your warranty at risk.
If it's been more than 18 months since your last professional clean, it's time. If you have pets and it's been more than 12, it's overdue.