To be honest, this is where most homeowners make their biggest mistake when it comes to carpet maintenance.
It's common practice to wait several years until your carpet barely resembles the original product before you call in the big dogs. If you want to maintain clean carpets, and extend the life of your carpets, you really ought to bring in a professional carpet cleaner on an annual basis.
The daily beating your carpet takes includes tracked in dirt, spills and stains, pet traffic, and even the presence of pest infestations such as fleas and mites. The more often you clean with hot water extraction, the less you'll have to worry about the presence of all these things as you watch your family spend time on your well maintained, and beautifully clean carpet.