How many opinions would you like? There’s no hard and fast rule that applies to all situations. Many factors such as children, pets, environmental situations and so on play a role in how quickly your carpet soils. Here’s some information to help determine the right interval for you.
The IICRC is an industry organization providing training and certification for cleaning and restoration professionals. The chart below is taken from IICRC Publication S001-1991, Standard Reference Guide for Professional On-location Cleaning of Installed Textile Floor Covering Materials.