Have you thought how many times a year you are having your haircut done. I guess anything between 1 week to 6 months. May be 4 weeks to 6 weeks to be very specific. (I guess this must be the average)
Althought it also depends on the kind of hair you have - if you have a healthy hair growth then definately you have to visit the barber every 1 month - because some of you hairs will definately from near you neck or may be ears and it would look odd. Yes people with longer hair might just have to trim their hairs.