A strained back muscle may take anywhere from a few days to several weeks to fully heal, depending on the affected area and severity of the injury. For the quickest recovery, stretching, exercises and other forms of physical therapy are recommended and will likely be included in a treatment plan for back strains. Simple techniques like stretching can increase the speed of recovery, helping moderate sprains heal in 4-5 weeks, with severe injuries taking around 10 weeks to sufficiently heal.