Whats a safe duration to be on Adderall (dextroamphetamine and racemic amphetamine)?

No limit. There is no artificial limit to how long a person can stay on Adderall (dextroamphetamine and racemic amphetamine). A person can stay on Adderall (dextroamphetamine and racemic amphetamine) into adulthood to treat the symptoms that persist from childhood.
As long as is useful. Length of time doesn't produce any particular problems, and there are people who have taken it for years, including some who are now elderly. Reasons for discontinuing it would include the independent development of illnesses for which it is a problem, e.g., if someone gets cardiovascular or neurological problems, and they can no longer take it -- but not length of tiime itself.