Hi, if one has just been diagnosed with adhd(inattentive) and started medication(concerta 18mg) how much improvement should one see?

Depends: If the label /dx is applied after exclusion of other issues that cause a similar lack of focus, 90% of patients should see improvement in days. The individual may not notice anything, but their improved focus & work output should be evident to supervising persons. A fraction need an alternative med. I am suspicious of any such label for an adult who was not found inattentive as a kid under 6.

It depends : Genetic factors play a role in response to adhd meds. Dosing can be titrated once a week until you are much better or you change your medication if increasing dose does not work