Depends on type of bichir, but you can expect somewhat fast growth for first year then slowing down after that. Like stated, depending on type, expect 12" to 24".
I have two, they max out at around 12". It is the smallest of the bichirs. It will hit around 7-8" in a year and slow down after that until it finally hits the max.
yeah depends on the species.. i've experienced 1" per month growth rate with my ornates and endlicheris til they get about 12" then it slows down considerably..