The leakage is proportional to the voltage as shown on the graph. Just pick the normal peak voltage the MOV will see in the circuit, and then look on the graph to determine the leakage. For example for 240VAC applied, the peak voltage is approximately 1.4 x 240 = 336V. Then looking along curve for the 625V MOV it can be seen that the leakage is below the left of the graph at 336V or <10E-6 (<1µA).