Hi! I recommend you read this book, it was the first one my dr told me about when I first got diagnosed, 5 years ago, it’s called Driven to Distraction.
Re: your questions, how many years you take medication depends on you and your decision, consult with your psychiatrist, because you may need to try a few different types before you find your best fit.
I haven’t had any side effects other than loss of appetite for the first month or two of taking Concerta, now I’m on Elvanse.
ADHD/ADD is a lifelong “Problem” but, therapy can help you deal with all the issues that came unnoticed until I got diagnosed with ADD, specifically anxiety and low self-esteem, I tried behavioural cognitive therapy and it worked wonders! Also, keep in mind that there are wonderful things that come with this diagnosis, like hyper focus, sensitivity and a sense of humour.