In comparison, maximum plasma concentration of Adderall XR is about 7 hours. Half life – When administering Adderall normally, the average elimination half-life for d-amphetamine occurs at 9 – 11 hours post dose vs. 11.5 – 14 hours for l-amphetamine. The mean elimination half-life for Adderall XR is about the same as for Adderall instant release, but varies according to age and body mass.