Few people will be convinced quickly, and few of those people who are convinced will have any immediate need for what you are selling.
By the time their need arises, your “short media schedule” will be over and they will have forgotten you.
You must decide what to say and then say it to as many people as you can afford to reach relentlessly.
If you cannot afford to reach a station’s audience with relentless repetition, you should consider buying a smaller audience.
It is better to reach ten people ten times than a hundred people once.
(Excerpt from Making Ads Work, chapter 4 'Testing the Advertising Waters')