If you have access to testing, you could try some out, and see how much toxin is absorbed in say a week, and then maybe test on a week to week basis.
In the lab I worked in a few years ago, we had activated carbon air filters, and they would need to be changed on a fortnightly basis, and sent to the manufacturer for disposal, or reactivation.
I would think that in the home, it would be much the same, possibly monthly, because the filters or charcoal wouldn't be absorbing as much toxin as in a laboratory.
I will have a scout around the internet, and see if I can find any further info on this.