Most carpet manufacturers recommend a professional hot-water extraction every 12 to 18 months, and many warranties require it. That's a good baseline for a quiet household with no pets and shoes left at the door.
Busier homes need more. With kids or pets, every six to twelve months keeps soil from grinding into the fibers, which is what actually wears carpet out. The gray paths in hallways and in front of the couch aren't just dirt on top; they're fibers being abraded by grit.
If anyone in the house has allergies or asthma, lean toward every six months. Carpet acts like a filter, holding dust, dander, and pollen, and a filter only works if you clean it out.
Between visits, vacuum high-traffic areas two or three times a week and blot spills right away with a white cloth and cool water. Skip the heavy store-bought spot sprays, which often leave a sticky residue that attracts more dirt.