3 I just put apple cider vinegar on my painful back sprain, how long does it take to heal? Dr. Ronald Galyon Dr. Galyon. Usually 8 weeks: Most back sprains heal in 8 weeks no matter what treatment is used as long as you do no re-injure it. This explains why so many methods claim to heal sprains. The main ingredient of acv is acetic acid. However, vinegars also have other acids, vitamins, mineral salts, and amino acids.