No, the 30/30 rule isn't the one about waiting half an hour before swimming after you eat. It also doesn't refer to the idea that anything you drop on the floor is still safe to eat if you pick it up within 30 seconds. The "rule" is similar, however, in that it's one of those pieces of advice that sounds sort of right and is easier to just pass along as accurate rather than investigating whether that's actually the case.
The rule is that you should start counting after you see a lightning flash. If you hear thunder in less than 30 seconds, lightning is sufficiently close that you should swiftly proceed indoors. Then wait at least 30 minutes until the storm ends to go back outside [source: Aleccia].
These days, experts say forget the counting. Those bolts from the blue and anvil lightning are difficult to anticipate. Instead, just go inside – or at least find a covered structure to hunker down under – at the first sound of thunder. Waiting 30 minutes after the storm is finished is still a good idea [source: Aleccia].