Ativan typically lasts from hours and is out of the system after 72 hours. The drug's effect will first be felt within 2 hours after administration, and the drug's. Apr 28, - Table 1. Benzodiazepine Equivalency Approximately Equivalent Oral Doses, mg Time to Peak Level, hours Half-life, hours Alprazolam (Xanax). Mar 10, - Peak concentrations in plasma occur approximately 2 hours following administration. The peak plasma level of lorazepam from a 2 mg dose is approximately 20 ng/mL. The mean half-life of unconjugated lorazepam in human plasma is about 12 hours and for its major metabolite, lorazepam glucuronide, about 18 hours.Ativan (Lorazepam) · Side Effects · B.i.d.