$T=2 \pi \sqrt{\frac{\mu}{k}}$

where,

$\mu=\frac{m_{1} m_{2}}{m_{1}+m_{2}}=\frac{2 \times 2}{2+2}=1 k g$

$=2 \pi \sqrt{\frac{1}{\pi^{2}}} \quad k=\pi^{2}$

$T=2 \sec$

Time for moving block remain in contact with

spring will be $\frac{T}{2} \Rightarrow \frac{2}{2}=1$ sec.

hence $t=\frac{T}{2}$ is $1$ sec.