An antibiotic was given in a dose = 300 mg single IV bolus dose, plasma concentration after 2 hr was (Cp2 = 5.5 mg/L), and after 6 hr was (Cp6 =4.7 mg/L). The drug follows one first order and one compartment model kinetics)

The drug is not effective when plasma concentration is 0.5 mg/L.the duration of action of 300 mg of this antibiotic is?