It depends on;
(1) the amount of use the carpet receives,
(2) the carpet type, (eg. wool or synthetic)
(3) your expectations of the carpet’s appearance.
It is important to clean the carpet before it begins to look tired.
In a family, home cleaning should be considered each 6 months to a year. In a busy commercial setting, this can be on a 2-3 monthly basis.
Leaving the carpet dirty for long periods of time will increase ware and reduce the lifespan of the carpet.