Taken from Marketing Without Megabucks . Just as in a public service announcement, a radio commercial must effectively reach listeners. You can be as simple or extravagant as you want, but keep the message focused and understandable. You are competing in the background with whatever the listener is doing, and people will not retain everything they hear.
Repetition allows the listener the chance to grab a pencil and jot down the details.
Repeating this information at the end cements the listener's identification of the product with your business, rather than with the generic idea of using the kinds of products or services you sell. In other words, if you run Heavenly Heat Hot Tubs, you want to not only tempt the listener into trying a hot tub, but coming to you, rather than a competitor. 30 seconds probably affords the best value-short enough to save money, but long enough to get the message across comfortably. Finally, get right to the point. There's no time for small talk in half a minute.