How long after an attack on a portal before a subsequent attack will trigger another notification?

I'm new to the game, but my understanding is notifications used to be sent every time a portal took damage. So, if an agent had to fire ten XMPs to completely take down a portal, ten notifications would be sent. I can see how that might be excessive, so a recent update changed it so that a notification is only sent when the first XMP damages the portal. Presumably, if enough time elapsed between the first and second XMP, another notification would be sent. I would like to know what that interval is. Clearly, if the interval between the first and second is only a few seconds, an additional notification is not sent, but if the interval is many hours, then an additional notification would be sent. Somewhere between there is a threshold.

This is relevant because the threshold can be exploited. For example, an agent planning to take down a portal could first fire a single XMP to trigger the notification, and then wait. The owner of the portal could respond by recharging the portal, but then they wouldn't see any further damage being done. Eventually they would probably give up defending the portal. The attacking agent could then wait for an amount of time less than the threshold and proceed with the actual attack. The owner of the portal would not receive a new notification that the portal was under attack.