Varies. A patient remains on a medication as long it is helping and causing no harm. 1/3 of children"outgrow" their adhd, usually by 16 yo, but sometimes not until mid 20's, then no longer needing medication. An adult with adhd will need to assess their side effects response & need for medication in consultation with the prescribing doctor. The usual symptoms of adult adhd tend to persist quite a while.
Really depends. Adderall (dextroamphetamine and racemic amphetamine) and other stimulants relieve adhd symptoms but do not change the underlying disorder. As children move into adolescence, 40-60% show less severe symptoms or develop better coping methods so may not need medications. Typically, adults who benefit from a stimulant will continue to need it unless they change activities to something where the adhd symptoms are not a problem.