Having done research on ADHD/ADD for about three years (a while ago, admittedly), this is still one of the hardest questions to answer for me.
It’s hard because ADHD/ADD is a difficult thing and because giving medication to treat behavioural problems (contrary to a genuine physical condition) is a difficult thing, even more so in children. The group is very, very heterogeneous.
I struggle with ADHD because it is still hard not to over- or underdiagnose these children. This is, IMHO, mostly due to the trend that DSM-IV research criteria, which are tentative and do not necessarily point to an established medical condition, but are used in everyday medical practice. This is a strange, if not a bad, thing.
Ritalin (methylphenidate) is, indeed, not as simple as speed, although it is an amphetamine. It may seem strange to give hyperactive children “speed” but the general idea is that they are hyperactive because the inability to focus or concentrate. Importantly, Ritalin also boosts the performance of non-ADHD children (cite). So of course (ADHD/ADD) children improve when administered Ritalin, but all children do, so that is hardly proof that some need it and some don’t.
Then there is the thing of adult ADHD. Technically, the DSM-IV criteria describe a childhood disorder. Nobody knows, at least as far as I know, how this develops into adulthood. Precious little reseach has been done in this area.
And, considering this, when to stop administering it? Short-term trials have found none or relatively mild side effect of Ritalin. But when this condition continues into adulthood and people could be dependent on Ritalin for life, that is a totally different thing. There may be unwanted negative side effects associated with long-term usage.
While this all points to a “no, don’t use the crap”, the alternative might be having your childhood, education and chances in life wasted because Ritalin wasn’t given to you when you did need it (for whatever reason; ADHD, stupid parents, name it) is, to say the least, kind of a bummer.
So there you go, I honestly wouldn’t know the answer to the OP, other than: be careful and decide on a child by child basis and try to keep it as short as possible.
And: always in combination with some kind of behavioural therapy.