This is a question I get asked quite often. A treatment course depends on each individual case. I typically start with a three time weekly treatment course utilizing spinal adjustments and physical therapies as needed. Depending on healing response, this rate of care could continue over the next two to four weeks. If there is significant initial healing, the treatment frequency is promptly reduced and/or stopped. So, the course of treatment really depends on the patient response. Often, patients are able to get improved healing with home therapies to include ice, heat or spinal stretches. Also, stopping the pain offending activities can help to speed up healing. For example, if a patient stopped sleeping on his/her stomach, this may help improve lower back or neck pain. Once significant and maintained healing response is noted, treatment frequency is typically weaned down to a release point, treating twice weekly, once weekly and then a final check in two to three weeks. If the patient maintains improvement, this reduction in care plan typically proves effective. Often, patients love the way they feel at the end of their treatment course and elect to continue with monthly maintenance treatments to avoid pain exacerbations and offset spinal stress.
But to answer this question in general terms, an average spinal sprain/strain injury typically takes four to six weeks to show significant healing response. Chronic pain often requires a protracted treatment course based on longevity of complaint and often underlying degenerative causes.
For further information on this topic, I found this a good read...
https://www.spine-health.com/treatment/chiropractic/chiropractic-treatment-program-guidelines