Are you talking about carbon in his filter? If he had detergent in the bucket, it would have had a pretty quick effect on the fish. And if had leeched into the plastic, it would take longer for it to leech back out tha just the few minutes the water was in the bucket for a water change. Also, carbon usually is only good for a day or so (some say a matter of hours) and then its fully saturated and not removing anything else. It can also slow a cyclin tank as it will pull ammonia from the water. That's basically carbon's job is to remove chemicals like ammonia from the water.
To be on the safe side though, I would get new buckets and mark them as fish buckets so no one uses them for soap.
Carbon stops working when it is saturated. Simple as that, there is no set time, if you have a lot of things to absorb it wont last long, if its sterilized, pure, sealed, airtight water, it will last for years. Plus you have to take into account of the quality of the carbon, how much water is going through it not around it etc. Plus carbon has never removed my ammonia, sponges have, but not carbon. Zeolite is the ammonia remover. Carbon probably does remove a tiny tiny bit of ammonia but it wont remove a noticable amount, at least not IME