Join Date: Oct 2017
Location: San Francisco, CA
My understanding is that Aquasoil works by absorbing the carbonates in your water (i.e., by reducing the KH) which reduces the pH.
For 11 months, my Aquasoil has been going strong, in the one tank I use it in. I expect, in the next few months, that it will lose its buffering capacity, and the pH will start rising from 6.6-6.8, and eventually settle around 7.2-7.4 (which is the water pH of my Walstad tank).
I anticipate the pH transition to happen over a couple weeks' time (but, obviously, YMMV).
BTW, there's no reason why you have to wait for the soil to become inert, before you take action. You can start replacing some of your tank's Aquasoil, in stages, over the course of a few months. Just vacuum out a small section, during each water change, and replace it with new soil. If you do it incrementally, you should be able to do a full turnover, without losing pH or risking ammonia spikes.
Just an idea.