Drug X becomes ineffective after 50 \% decomposition. The original...

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