I had a lidocaine injection for middle back pain which provided much relief. forgot to ask how long it will last.

Few days : Not sure exactly what kind of injection you had. Most injections with local anesthetics only last hours to days. There us some research that shows that they can last a month or longer. Tough question to answer.

Back Pain: Depends on whether you had an epidural steroid injection or a "trigger point" injection. The duration could be anywhere from months to years based on the condition being treated and if you reinjure the area. If you have a herniated disk it will probably be a reoccurring problem. If all you had was muscle spasm from physical activity them it should be permanent relief. Learn how to avoid injury.