Probably fairly consistantly since 8th/9th grade, mostly coupled with football and track... about 13-14 years now? (Also, before that I did about 2 years on the small bench/barbell in the garage, and a little bag work back then too with a uncle of mine) sheesh, I feel old.
I took a couple months off every few years early on, but the last 4 years I have been slacking off but still hitting the gym at least 2-4x a week.... I have been trying to hit it hard again (and smarter) for maybe the last 6-9 months...