When I decide it's too long. Probably averaging about every 2 weeks, at a rough guess. I do it myself, shaver on 2 and go all over my head, job done. Like Street, my hair grows quickly but not all over my head. I should probably do it more often and I would if I cared more about my appearance. But I care so little that I don't even own a mirror. I don't look at me, so why should I care much what I look like? If it's adequate for work, fine.
I used to have waist-length hair. That was a PITA to take care of. Hours of washing and conditioning and drying. Now I just wash my hair as a small part of my shower and give it a quick rub with a towel to dry it.