Time $t=CR$ is known as time constant. It is time in which charge on the capacitor decreases to $\frac{1}{\mathrm{e}}$ times of its initial charge (steady-state charge).

In figure (i) PN junction diode is in forward bias, so current will flow in the circuit i.e., charge on the capacitor decrease and in time t it becomes $\mathrm{Q}=\frac{1}{\mathrm{e}}\left({\mathrm{Q}}_{\mathrm{o}}\right)$ ; where ${\mathrm{Q}}_0=CV\Rightarrow \mathrm{Q}=\frac{\mathit{C}\mathit{V}}{\mathit{e}}$

In figure (ii) P-N junction diode is in reverse bias, so no current will flow through the circuit hence change on the capacitor will not decay and it remains same i.e. CV after time t.