Short answer, I think 28 days or 4 weeks is a decent period of time to have in mind whenever you have any doubts whether the dermopathy is completely gone. This is because epidermis regeneration cycle takes that long.
Understanding the life cycle of skin | Galderma
(this is random source that came on top after a google search, but I know this fact and Galderma is a biotech company involved in dermatology so they can misinform their investors)
From my experience the dermopathy always comes in waves. It always starts with slightly dry lips and eyes, and if you don't watch the kava dosage, face symptoms worsen a lot and then go all the way down to feet in a wave, ie you face and neck can improve, while the lower body skin is in its worst. It is an amazing condition in itself, especially since it's completely reversible.
So drinking kava has an accumulative effect. The amazing part here is that the single dose you had, in my thinking, has still lingering negative effects on skin days if not weeks after you drank it. Personal sensitivity is probably a major factor here as somebody develops full blown dermopathy after three weeks of daily consumption and somebody after six months. Reading comments on this forum seems like experienced drinkers find a sweet spot in terms of kava effects and side effect (I'm not there yet after three years of drinking it). This is a good thing in a away, as what else would stop you drinking yourself to oblivion. If kava would have any less side effects and god forbid, it would taste like Twinings, then global supply of kava would be consumed by a single village in the UK.
I had episodes when a new kava dermopathy "wave" would start even after two weeks after I stopped drinking. But I could tell it's kava doing its work, as it would also start in the same pattern, but in a much milder way. So, you can find your self that you feet look like you described, torso is just fine, but you start noticing new dry lips and itchy eyes. After two weeks!
I am not saying you need to wait for the full 4 week cycle. I am also not claiming that kava specifically targets epidermis, I have never looked into this question in detail. But you're on your way to look better soon. One conclusion I made, that if you assume that it has a cumulative effect, then stopping altogether is the best option to reverse the dermopathy as soon as your body allows. But this is not necessarily the only option. I have been able to manage dermopathy by significantly cutting the dose, like 1 or 2 table spoons per day, then a day off, then little bit again. But this approach is prone to you relapsing back to your usual kava drinking pattern, so a total break is still the best option (for other reasons as well like GGT and ALP normalisation, if that matters is another question).