Drug X becomes ineffective after 50 % decomposition. The original concentration of drug in a bottle was 16 mg/mL which becomes 4 mg/mL in 12 months.…

Drug $\displaystyle X$ becomes ineffective after $\displaystyle 50 \%$ decomposition. The original concentration of drug in a bottle was $\displaystyle 16$ mg/mL which becomes $\displaystyle 4$ mg/mL in $\displaystyle 12$ months. The expiry time of the drug in months is $\displaystyle \_\_\_\_$. Assume that the decomposition of the drug follows first order kinetics.