Very quickly to around 10", can be up to an inch every 3 to 4 weeks. Once they hit the 10" expect a very reduced growth rate until it max's between 12 and 13".
Of course diet, genetics and tank/water conditions will influence the above.
It varies from Bichir to Bichir. Mine grew quickly until 6" and it's probably 6.5" now after a couple of months after the 6" mark. And I haven't seen any growth on my WC Delhezi.