Run a search on the average length of a sound bite, and you will see that it lasts anywhere from 5 to 15 seconds. So let's split the difference and say the average is 10 seconds in length. (This is in sync with an NPR article on The Incredible Shrinking Sound Bite, which has it at 9 seconds, but 10 is easier to remember.)
Well, that is about 21 spoken words.
Key messages should be short & snappy.
How many key messages can you speak in 21 words? Probably 2 if you are using full-length sentences (which you should).
So, 2 key messages; 21 words; 10 seconds. 2, 21, 10.
"The Smithtown Police Department arrived in record time after the homeowner called 911. We urge all residents to lock their doors."
"The festivities will kick off at 2 pm on Saturday! Come on down to Main Street and take part in the fun!"
"These charges are outrageous, and their only intent is to sully my good name. I will have my day in court."
See how easy? Be prepared to start with your key messages, and then if you have more time, be prepared for secondary messages that follow the same formula.