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1.
Taking 30 seconds to think before you say anything, because 99% of what you say right away is stupid.

This is most commonly used in the World of Warcraft.
"Raloth, 30 second rule!!!"

"Shut up Raloth, 30 second rule!"
by Valjean September 15, 2008
 
2.
In the case of not knowing exactly how long to keep smaller portions of food in a microwave oven, the general rule of thumb is to tap the "+ 30 Seconds" button on your microwave.
How long should I put the quesadillas in for?" "I dunno, 30 second rule?
by Frohman23 September 06, 2010