Recall the formula of current in terms of angular frequency, 
$i=i_0\sin \omega t$, here the maximum value is, $i_0=i_{RMS}\times \sqrt{2}$,

here, the value of $\omega$ is the same as, $2\pi f$, and $f=50 \mathrm{Hz}$, given in the question, 

$$\begin{gathered} \Rightarrow \frac{i_0}{\sqrt{2}}=i_0\sin \omega t\Rightarrow \omega t=\frac{\pi }{4} \\ \Rightarrow t=\frac{\pi }{4}\times \frac{T}{2\pi }=\frac{1}{8\times f}=\frac{1}{8\times 50}=2.5 \mathrm{ms} \end{gathered}$$