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.