Maximum acceleration of the system $\left(a_{\max }\right)=-\omega^2 A$
For a block to escape the board the acceleration must be equal to 9 at the top-most point.
$g=\omega^2 A$
$\omega=\sqrt{\frac{g}{A}}$
Time period $=\frac{2 \pi}{\omega}=\sqrt{\frac{A}{g}}$

