Every three or four years. I'll buy a new six-pack of underwear every once in a while, and I'll buy a new pair of pants when an old pair wears out. But other than that, I only really buy clothes when I get a new job with a new dress code. I still wear many shirts and pants from high school.
Clothes shopping is really bad for my self esteem, and puts me in a rotten mood, so I avoid it as much as possible.