Usually you can tell when it's time - when it no longer has the intended effect of keeping the ADHD under control. If one month your child is focused and working well at school and socially doing well - then next month things begin to crumble then it's time to talk about what to do.
Make sure your child gets some retraining using therapy. With therapy as your child gets older you can make sure he doesn't fall back on old and unwanted habits that tends to happen to all with ADHD. By knowing there are other ways to behave and work - he'll be informed and ready for the day he needs it.