For me it was very obvious on day 1, both on Xaggatin (an offbrand concerta copy) and ritalin prolonged release. There was just a feeling of a bit more energy, clarity and drive - it was kinda how I felt on a really good day premeds, if I had a day where I was cheerful and excited about something, then the adhd symptoms weren't as bad as usual. On my meds, that's basically everyday now.
But I was on concerta for a while in between the other two, and found that they did very little for me. Even though Xaggatin and concerta are meant to be the same, and I was on the same dose, the concerta didn't really work for me at all (despite being more expensive!).