Hair length per se is irrelevant to the demand for haircuts. What matters is the acceptable range of hair length - roughly speaking, the difference between the shortest acceptable length and the longest acceptable length. If your hair grows an inch per month, and the acceptable length ranges from 2 to 4 inches, you need a haircut every two months. If the acceptable length ranges from 21 inches to 21.5 inches, you need a cut every two weeks.
As one reader observed, however, it is likely that acceptable length and the range of acceptable lengths are correlated. When short hair is in fashion, you have to stay between, say, .5 and 1.5 inches. But when long hair is in fashion, you have more latitude - anything between say 10 and 14 inches is OK. As a practical matter, then, demand for haircuts is higher when the fashionable length is short.