For a given reaction of first order, it takes 20 minutes for the concentration to drop from $1.0 \mathrm{mol} {\mathrm{litre}}^{-1}$ to $0.6 \mathrm{mol} {\mathrm{litre}}^{-1}$. The time required for the concentration to drop from $0.6 \mathrm{mol} {\mathrm{litre}}^{-1}$ to $0.36 \mathrm{mol} {\mathrm{litre}}^{-1}$ will be