On average, how long will a lower back strain last?

Sprains and strains: Muscle pain in the neck or back after over-exercise or strain usually worsens the first couple days. Thereafter, symptoms usually improve daily. Healing time is proportional to the injury—minor strain should heal in a couple days, while a severe strain may take 6 weeks or more if a muscle tear resulted. So just ease back until pain subsides. That's your clue to rev back up.