Rugs under normal use should be cleaned at least every 18 months - and if there is high traffic, high dust allergies, lots of kids, lots of pets - it should be annually.
If they have a regular dusting routine with their textiles (weekly with a canister or other non-beater bar vacuum) or rugs in low traffic areas - they can extend it to 2-2.5 years.
The problem for rugs is the continual layer of dust that settles on and into the fibers over time. This causes the abrasion to the wool, and this is not simply dust, it's other contaminants as well.
So - how often it is cleaned depends on how clean they keep their house. But 18 months is what we recommend.
Lisa
www.TheRugChick.com