I agree that the success rate could be much lower, as well as your point about the meds. When treatments are not successful, people will often seek further help elsewhere without necessarily informing the last treatment provider. I think he believes what he is saying, but doesn't necessarily know about some of the inevitable harder cases that continued elsewhere.

But regarding the duration of treatment, I'm skeptical that there is some natural law, rule or trend that it is the same length as the duration of the symptoms. I myself am getting about 10% better which each chiro visit, so hopefully I'll be a living example.

One other thing I'd like to mention about getting chiro treatment, provided your chiro has intelligence and skills, is that you may find that you have additional issues. Not only is it worth getting them resolved, but these issues can interact with each other. Getting an expert to identify the network of problems and break it up has its merits. Other experts may be suitable for this as well including DOs and PTs.