Ideally, once every 2 to 2 1/2 months. Sometimes I go three months before I get a haircut. The longest I've gone in the last several years is four months.
Right now I'm playing around with my hair so it's completely random. But usually if I'm keeping the same hairstyle, once a month. If I had the money I'd get it cut every 2-3 weeks.
Once every six weeks and now its gone up from $25 to $28 merde, worse both of the barbers here are only part time and god knows where I am going to find a decent barber that doesn't cost once they retire.