Strange that they seem confused -- it's a common thing hairdressers should know about!

My hair is thick and grows fast, and I really like it to be really short (I get a #1) so ideally mine would be done every four weeks or so (in reality it's often more like 6 or 8 by the time I get around to it but I think it's pretty untidy by that point). If your hair is finer or you don't want it quite that short or don't mind it getting a little longer, you could leave it longer between haircuts.