The A320 is certified to have approx 60mins on emergency battery supply.
Trying to start the APU on batteries alone reduces this time ( by approx 3 mins, I think..to be corrected if so, for every attempted start) so you'll want to be within 60 mins of the nearest suitable airport before things go dark.
One sim check involved DC 1+2 FAULT which meant you couldn't start the APU because the batteries were not available-no DC BAT BUS
