So you forget to take it for a couple of days? The biological half-life of dextroamphetamine is 10 hours and levoamphetamine 13 hours. That's 10 to 13 hours (since Adderall is a mix of both isomers) until just half of the dose you took is eliminated from your bloodstream. If you take something with a long half-life like that every day, it's constantly in your system. It takes roughly three days to completely eliminate a dose.