When something is far-reaching or expansive it is extensive. When your friend knows every single place and time that his favorite band has played any given song, he has extensive knowledge of that band.
Use the adjective extensive for something that goes on and on and on. It works for anything that covers a lot of territory either in physical space, as in "the gardens at the palace are extensive," or conceptually, as in "her knowledge of the mortgage-backed securities market is extensive." If you are a farmer, you would also use extensive to mean that you are able to work a lot of land without too much effort or manual labor.