Research on humans suggest that active hair fiber growth production from scalp hair follicles may last from 500 to 1800 days. The scalp hair fiber growth rate is between 0.3 to 0.35mm a day. For the most part, the studies were done with male volunteers, but a few other studies indicate similar results for women too. However, from the picture below you can see that some people are able to grow their hair much longer. The upper limit to scalp hair length is determined by our genes.
Some people have genes for short hair, others grow hair much longer.
Ageing and hair cycles. Br J Dermatol. Periodicity in the growth and shedding of hair. Br J Dermatol. Saitoh M, Uzuka M, Sakamoto M. Human hair cycle.