Here is a link on Stainmaster carpet saying how often you should get your carpets cleaned and how to take care of your carpets.
If you go to Stainmaster website it says you should get your carpets cleaned every 18 months or it will void the warranty of the carpet. So in that case, I think you should get your carpet cleaning done every year. It is just fabric like your clothes they just need to be washed once in a while if they look dirty or not. They still need to be cleaned, because some of the odors and small dirt you might not see, but getting them cleaned every year will keep your carpets healthy and make them last much longer. The best thing you can do is vacuum your carpet regularly to get all the dry soil in the carpet that might scratch and damage the fibers of your carpet. The carpet in your home is the biggest filter in your home and it helps with the air quality by not allowing the pollen, mold spores, dust and dirt floating in the air of our home. Regular vacuum cleaning and carpet cleaning will keep your carpets healthy and safe for you and your family