It seems to be a bit of a lottery.
DD (14) is on methylphenidate. We started on 5mg twice a day of the instant release back in June, then moved to 10mg twice a day after a week. Started Equasym XL 20mg 4 weeks ago.
We got pretty instant results straight away in terms of it helping with the hyperactivity symptoms, but it wasn't till she started the Equasym XL that we started to see an effect in terms of focus in class. Seeing the consultant in a few weeks and DD is keen to try pushing the dose up a bit more.
When I've asked her about it, she just says she's less fidgeting, more one thought in her brain and that geography and physics are more interesting than she thought now she actually focuses. But it's not as if she's suddenly taken a magic pill and facts are at her finger-tips - more that she doesn't treat the topic they've studied for last three weeks as something completely unheard of when she has to do homework.
One of her cousins has had a similar response and was doing well on Concerta 20mg for a number of years. Now looking at a dose increase.
However two of her other cousins (yay for genetic inheritance) did not get on with methylphenidate at all. One hasn't found anything that works, and the other is finding Strattera works well.
So, it may be that it's not the medication for her, or the dose isn't right yet, or her expectations are too high. I believe in the US they're now using tests that can pinpoint the medication you are most likely to respond to.
Definitely recommend the book a PP suggested. It's incredibly helpful and useful.
Might also be worth trying a different brand of methylphenidate - some have different amounts of instant and slow release percentages, and sometimes you just get on better with a particular brand.