Ian
Not sure how to quantify carpet drying without setting up an experiment, taking moisture readings etc. However I can say when we have used them on flood jobs, placed on a concrete floor slab the area in front of the fan is surface dry in about 15 minutes. Surface dry means the surface moisture has evaporated, obviously the concrete slab is still wet.
On carpets I would imagine the only area being dried was that which has direct airflow going over it with slight drying taking place on the rest due to increased air movement in the room.