How long will a bacteria infection on the torso usually last for?

Depends on… The severity of the infection (how large an area involved, how deep) & what organism (s) causing the infection & of course, whether it is being treated & how aggressively. Other factors are comorbid conditions like diabetes, especially if poorly controlled. Minor infections caused by run-of-the-mill bacteria, treated with appropriate antibiotic, usually take 7-10 days. More complicated take longer.