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