It depends on a number of factors:
1) type and quantity of food the bichir gets. Obviously, the better the quality of the food and the more it eats...the faster it will grow.
2) the conditions under which it is kept. Regular partial water changes to maintain water quality and plenty of room will be beneficial.
3) the bichir itself...genetics always makes a difference. A six inch P. ornatipinnis is a young one so it should be actively growing.
That being said...I've seen Polypterus ornitipinnis gain two inches in length in less than a month with a good diet and good water quality.