any idea how long it would take to turn the soil into muck??
The H2O2 isn't what turns the AquaSoil (AS) into muck. It attacks organic matter only. As far as I know, AS is completely inorganic, just clay enriched with nutrients and mildly baked.
But I know for certain it's designed to slowly break down, releasing nutrients into the water so plants can use them, and slowly turning to mud in the process.
I believe the concern zico_aqua is expressing is only that excessive handling, such as during a gravel vac, will accelerate the breakdown of the soft AS and may generate too much mud.
It's pretty unlikely there's still H2O2 left in the AS. It should have diffused out and been removed during the multiple water changes. It could be tested for sure by adding a bit of potassium permanganate to the water, and any H2O2 leaking out will neutralize the pink color. But I think you've had enough strong oxidizers in your tank without adding more!