Using the Definition of Impulse, we have:
[tex]\Delta Q=\Delta I \\ mv=F\Delta t \\ 12*10^2*10*kg*m/s=6*10^3*\Delta t *N \\ \Delta t= \frac{12*10^3}{6*10^3} \frac{kg*m}{s* \frac{kg*m}{s^2} } \\ \boxed {\Delta t= 2s}[/tex]
If you notice any mistake in my english, please let me know, because i am not native.