A resistance of 40   Ω is connected to a source of alternating current rated 220   V , 50…

The current in AC is of the form of $i=i_0\sin \omega t$. The maximum current is $i_0$ and rms current is $\frac{i_0}{\sqrt{2}}$. The change in the phase when current change from maximum to rms value is $\frac{\pi }{4} \mathrm{rad}$. Therefore, the time taken for the same will be,

$t=\frac{∆\varphi }{\omega }=\frac{\pi }{4\times 2\pi \times 50}=2.5 \mathrm{ms}$.