Answer
When $\Delta G=0$, that is, when the system is in equilibrium.
Work Step by Step
Recall that $\Delta G= \Delta H-T\Delta S$
For $\Delta S=\frac{\Delta H}{T}$ to be true, $\Delta G$ must be 0.
$\Delta G=0\implies $ system is in equilibrium.
Therefore, $\Delta S=\frac{\Delta H}{T}$ is true when the system is in equilibrium.