That is what happens whenever the description in the sentence is incomplete.
The Peak Inverse Voltage (PIV) of a diode in a full wave voltage doubler , which is what you are showing here, is the sum of the stored voltage in the capacitor (Vm) plus the AC's negative peak voltage (Vpk-).
Since for such a high voltage, the diode's forward voltage drop can be disregarded, then one can safely simplify Vm = Vpk-
We further know that in an ideal sinewave Vpk = sqrt(2)*Vrms.
Thus for your 250 volt AC source, the total inverse voltage the diode will see is 2*1.41*250 = 705 volts.
Applying some safety margin, you should use 800 PIV diodes.