But hair is still made up of discrete quantity of "stuff"
Well, eventually you get to a point where hairs grow quickly enough that the haircuts effectively stop. I think the sum total of hair material you trend towards at the limit would be the sum of length of hair that grows during the time it takes to do a haircut. Whether it's 1x or 2x of the hair growth amount depends on whether you measure at the start, or end, of the haircut