There is a lot of variables , a quick answer is anything from 5 to 12 hours
This plays a large part in how long , a natural fibre tends to take longer due to holding more moisture than man made fibres.
Heavily soiled carpets
With heavily soiled carpets more pre- spray will need to be used, and more passes with the the hot water extraction machine will be need to flush away the soiling.
If its raining the it will take longer due to the atmosphere being full , and on a nice breezy dry day not as long .
What you can do
Opening windows , Turning your heating on , Turning on fans , Opening doors on both side of your home to cause a through draught and good airflow will all help to shorten the drying time
A professional carpet cleaner will bring large floor fans with him to aide faster drying times .
Never close all your doors and windows and leave the house for long periods when the carpets are not dry , as this can cause mould and smell problems
By Neil Kelloway