Uhh, can't say how long they last, 'cuz mine certainly aren't going away any time soon. I can commiserate, though.
Nearly five months on, and my back is a downright mess; so was my dad's when he went through puberty. My back didn't fare so well the first time through puberty, either, but at least the prescription cream I had helped (very effective the first time around, but unfortunately also extremely expensive--about $80 for a small tube). But it's not doing much on this T-inspired acne, I'm afraid. The tight sports bra is not helping matters, either >.<
So yeah, 14-year-old zitty boy... I'm currently rocking that look out.