I can't speak to the Airbus specifically but 30 min is a typical design standard consideration. Generally, if the OEM bookwook doesn't quote a figure it would be reasonably safe to presume around 20-40 minutes depending on load.
It is for just this consideration that some of us don't do the typical sim electrical failure - reposition - standby power ILS .. far better to have crews faced with the time management problems of the aircraft recovery.
Generally it only takes one run to get the message across ...