Carpet manufacturers usually say every twelve to eighteen months, and most warranties require professional cleaning at some interval to stay valid. That is a floor, not a target. The right number for your house depends almost entirely on what walks across the carpet.
Two adults, no pets, shoes off at the door: eighteen months is genuinely fine. Add a dog and it moves to about a year. Add kids and a dog, or a household where shoes stay on, and six to nine months keeps the carpet ahead of the damage instead of catching up to it.
Coastal homes are their own category. In Point Loma, Ocean Beach, Coronado, and along the North County coast, fine sand gets tracked in constantly and works its way to the base of the pile. Sand is abrasive. Every step grinds it against the fibers, and that abrasion is what actually wears carpet out — not the visible dirt on top. Beach-adjacent carpet that never gets deep cleaned wears out years early even when it doesn't look especially dirty.
The useful test is not how the carpet looks from standing height, it's what happens when you rub a damp white cloth firmly across a traffic lane. If the cloth comes up grey, there is soil down in the pile that vacuuming is no longer reaching, and that is the point where cleaning is maintenance rather than rescue.
One thing worth knowing: cleaning carpet does not make it get dirty faster. That myth comes from cleaners who use too much detergent and don't rinse it out, leaving a sticky film that grabs every bit of soil that lands on it. Done properly, with a thorough rinse, clean carpet stays clean longer than dirty carpet does.