The wood charcoal would last a good bit based on research done, and for sure the activated carbon/biochar would last many many years also based on research done.Just got my lab results back the other day, haven't had time to look at them in detail but looks like phos is creeping up in the 3x3. I need to take a closer look before doing anything. Shit I may not be planting anything in there till fall anyway.
I'd think the charcoal would break down pretty quickly? I really like the wood mulch (vs. rice hulls) and have enough chipped up to last a while. Still need to work something in and that something will probably be pumice but I have half a bale of rice hulls too.
Biochar is one of the big things theyre now using for adding to the good potting mixes.
Heres an interesting research paper on it, from a reputable source, giving a good rundown on things. I'll be taking the AC and putting it in some distilled water to do a pH slurry test to see where what I have sits at. Should give me an idea what it may change the soil to as it has a liming effect it can impart.
After getting a read on where the pH is sitting I'll inoculate it with some compost/worm poop tea. Then mix up a pot with it and put a known clone in there and see what she does for me. Be a good little test imho.
I've got a bag of pumice as well like you'd mentioned i'll throw in the mix as well. Prob do some perlite too since thats an easy known solution and its cheap to grab.