My son in law is taking co-codamol 8/500 following a car accident for back pain. how long can he take this for safely?

Not long: Co-codamol is comprised of Codeine and paracetamol (acetaminophen or tylenol equivalent in the usa). Codeine is addictive at some point and may not even be the best solution for his pain. In the uk i think the 8/500 is available without a prescription but many patients pile on the dosing and get into trouble more quickly. If it has been more than 3-5 days, suggest he see a health care provider.

Few days : If the injury is severe, foe e.g., imaging study shows large disc herniation, and if he has tried other treatment measures, maybe okay. But generally want to treat back injuries without opiates as it is associated with greater disability. It can lead to dependance as well making it very very hard to get off of the medication. There are better options than co-codamol.