Answer
$7.0ms$
Work Step by Step
We know that
$\Delta p=F_{avg}\Delta t$
This can be rearranged as:
$\Delta t=\frac{\Delta p}{F_{avg}}$
$\implies \Delta t=\frac{m(v_f-v_i)}{F_{avg}}$
We plug in the known values to obtain:
$\Delta t=\frac{(0.50)(3.2-0)}{230}$
$\Delta t=7.0ms$