Methylphenidate efficacy was immediate for me. Try a couple of tests. List a few things that you found very difficult and check your performance about 2 hours after taking your Concerta. Many Concerta users have reported "it is very slow out of the gate." That is why to wait at least 2 hours before testing. My guess is that you'll see a noticeable improvement. If not, the dose may still be too low. Or, another disorder or comorbid disorder is at work. Or, conceding that we all respond differently, stimulants may indeed take a few weeks for some folks.
Bob