737NG Pilot, never seen that delay on ground on A320. There is max 2 min cooldown cycle before APU shuts down (depends on bleed usage). Inlet flap closes immediately after APU rpm is less that 7%. Where did you find information about 15 minutes delay and could you please insert a quote from a document here? Could you also tell what do you mean under automatic shutdown? There is no automatic shutdowns on APU - it is either normal, protective or emergency.