According to the newton's second law of motion, the force is equal to the rate of change of momentum. i.e, the ratio of change in momentum to the time interval.

Force is given by, $F=\frac{∆P}{∆t}$

$\Rightarrow ∆t=\frac{∆P}{F}=\frac{0.4}{2}=0.2 \mathrm{s}$