That is pretty variable I would think due to ground composition and how complete the char burn is. In my anecdotal experience, if you do a low grade charcoal that still has a decent amount of oils in it like a lot of old-school burn wood and bury it in earth or drench in water, most of it will have broken down to invisibility after just a few years or so mixed into the ground. However charcoal that I have burned in a container sealed from air ingress and using a secondary source of heat until the wood stops venting any gases and is nearly pure carbon, I still see decent size chunks of it in my garden areas over 2 decades later, and no reason to expect it to not still be there even 50+ years from now.

The current best in class methodology for biochar aims for 100+ years. There are some folks saying there are pathways to make biochar last 1000+ years.

Puro methodology: https://biochar.groups.io/g/main/attachment/32853/2/Puro.ear...

The nice thing is biochar is relatively inert. It just sits there in soil, holding onto water, making space for organisms to grow, but isn't "food", so it doesn't get eaten up and turned back into carbon dioxide. So it's a win for farmers and carbon removal.

7 to 10-ish years from what I recall. Which is another thing that makes it so low risk. If you stop the subsidy it goes away with a few years.