Hair grows at a rate of about 1 centimetre a month.
(But it can only grow so long, and that is down to the individual and their age. Typically, waist-length hair is 80-90 cm long, and will have taken about seven years to grow, but as you grow older the growth period of hair drops to about three years, so your hair will only grow to about shoulder length before it falls out or is brushed out. I never realised hair was so fussy!)